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.

share/org.logdiag.gschema.xml

@@ -1,5 +1,5 @@
 <schemalist>
-	<schema id="org.logdiag" path="/apps/logdiag/" gettext-domain="logdiag">
+	<schema id="org.logdiag" path="/org/logdiag/" gettext-domain="logdiag">
 		<key name="show-main-toolbar" type="b">
 			<default>true</default>
 			<summary>Whether to show the main toolbar</summary>