Bump the CMake version requirement, fix Windows

- Add a missing runtime library for gettext-tools.
 - Remove the no longer necessary customized FindGettext.cmake,
   since the downloaded gettext binaries are new enough.
 - Make the installer at least ask to uninstall previous versions.
 - Adjust the PATH of tests so that they'll run on Windows directly.
 - Fix quoting so that the project will build inside paths with spaces.
 - Resolve a GSettings deprecation warning.
 - Update the README's build instructions as appropriate.
 - Require CMake 3.9 because of the README's suggestion to fix
   FindPkgConfig.cmake manually using separate_arguments(UNIX_COMMAND).

Tested build configurations:
 - native Arch Linux,
 - native Windows XP with the newest NSIS,
 - Arch Linux Mingw-w64 i686 Win32Depends.cmake NSIS cross-build + XP,
 - Arch Linux Mingw-w64 x86_64 MSYS2 NSIS cross-build + Windows 10.

Detected issues:
 - The file save dialog will not add the extension automatically,
   seen with MSYS2.

cmake/FindGettext.cmake

@@ -1,99 +0,0 @@
-# - Find GNU gettext tools
-# This module looks for the GNU gettext tools. This module defines the
-# following values:
-#    GETTEXT_MSGMERGE_EXECUTABLE: the full path to the msgmerge tool.
-#    GETTEXT_MSGFMT_EXECUTABLE: the full path to the msgfmt tool.
-#    GETTEXT_FOUND: True if gettext has been found.
-#
-# Additionally it provides the following macros:
-# GETTEXT_CREATE_TRANSLATIONS ( outputFile [ALL] file1 ... fileN )
-#    This will create a target "${PROJECT_NAME}_translations" which will
-#    convert the given input po files into the binary output mo file.
-#    If the ALL option is used, the translations will also be created
-#    when building the default target.
-
-#=============================================================================
-# Copyright 2007-2009 Kitware, Inc.
-# Copyright 2010-2011 Přemysl Eric Janouch
-#
-# Redistribution and use in source and binary forms, with or without
-# modification, are permitted provided that the following conditions are met:
-#     * Redistributions of source code must retain the above copyright
-#       notice, this list of conditions and the following disclaimer.
-#     * Redistributions in binary form must reproduce the above copyright
-#       notice, this list of conditions and the following disclaimer in the
-#       documentation and/or other materials provided with the distribution.
-#     * Neither the name of the Kitware nor the names of contributors may be
-#       used to endorse or promote products derived from this software
-#       without specific prior written permission.
-#
-# THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
-# AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO,
-# THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
-# PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDERS BE LIABLE
-# FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
-# DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
-# OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
-# HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
-# LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
-# OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY
-# OF SUCH DAMAGE.
-#=============================================================================
-
-find_program (GETTEXT_MSGMERGE_EXECUTABLE msgmerge)
-find_program (GETTEXT_MSGFMT_EXECUTABLE msgfmt)
-
-macro (GETTEXT_CREATE_TRANSLATIONS _potFile _firstPoFileArg)
-	# Make it a real variable, so we can modify it here.
-	set (_firstPoFile "${_firstPoFileArg}")
-
-	set (_gmoFiles)
-	get_filename_component (_potBasename ${_potFile} NAME_WE)
-	get_filename_component (_absPotFile ${_potFile} ABSOLUTE)
-
-	set (_addToAll)
-	if (${_firstPoFile} STREQUAL "ALL")
-		set (_addToAll "ALL")
-		set (_firstPoFile)
-	endif (${_firstPoFile} STREQUAL "ALL")
-
-	foreach (_currentPoFile ${_firstPoFile} ${ARGN})
-		get_filename_component (_absFile ${_currentPoFile} ABSOLUTE)
-		get_filename_component (_abs_PATH ${_absFile} PATH)
-		get_filename_component (_lang ${_absFile} NAME_WE)
-		set (_gmoFile ${CMAKE_CURRENT_BINARY_DIR}/${_lang}.gmo)
-
-		# msgmerge versions older than 0.11 don't actually support --update
-		# and --backup, let's try to workaround that (tested on 0.10.40).
-		execute_process (COMMAND ${GETTEXT_MSGMERGE_EXECUTABLE} -V
-			OUTPUT_VARIABLE _msgmergeVersion)
-		string (REGEX MATCH "0[.][0-9]+" _msgmergeVersion ${_msgmergeVersion})
-		if ("${_msgmergeVersion}" MATCHES "[.]10|[.][0-9]")
-			set (_msgmergeParams --quiet -s
-				${_absFile} -o ${_absFile} ${_absPotFile})
-		else ("${_msgmergeVersion}" MATCHES "[.]10|[.][0-9]")
-			set (_msgmergeParams --quiet --update --backup=none -s
-				${_absFile} ${_absPotFile})
-		endif ("${_msgmergeVersion}" MATCHES "[.]10|[.][0-9]")
-
-		add_custom_command (
-			OUTPUT ${_gmoFile}
-			COMMAND ${GETTEXT_MSGMERGE_EXECUTABLE} ${_msgmergeParams}
-			COMMAND ${GETTEXT_MSGFMT_EXECUTABLE} -o ${_gmoFile} ${_absFile}
-			DEPENDS ${_absPotFile} ${_absFile}
-		)
-
-		install (FILES ${_gmoFile} DESTINATION
-			share/locale/${_lang}/LC_MESSAGES RENAME ${_potBasename}.mo)
-		set (_gmoFiles ${_gmoFiles} ${_gmoFile})
-	endforeach (_currentPoFile)
-
-	add_custom_target (${PROJECT_NAME}_translations ${_addToAll}
-		DEPENDS ${_gmoFiles})
-endmacro (GETTEXT_CREATE_TRANSLATIONS)
-
-include (FindPackageHandleStandardArgs)
-FIND_PACKAGE_HANDLE_STANDARD_ARGS (Gettext DEFAULT_MSG
-	GETTEXT_MSGMERGE_EXECUTABLE GETTEXT_MSGFMT_EXECUTABLE)
-
-