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.

Win32Depends.cmake

@@ -28,8 +28,12 @@ set (pkg_7za_urls "${pkg_7za_root}/7-Zip/9.20/7za920.zip")
 set (pkg_7za_md5 "2fac454a90ae96021f4ffc607d4c00f8")
 
 set (pkg_gtk_root "http://ftp.gnome.org/pub/gnome/binaries/win32")
-set (pkg_gtk_urls "${pkg_gtk_root}/dependencies/gettext-tools-0.17.zip")
-set (pkg_gtk_md5 "09baff956ebd1c391c7f71e9bd768edd")
+set (pkg_gtk_urls
+	"${pkg_gtk_root}/dependencies/gettext-tools-0.17.zip"
+	"${pkg_gtk_root}/dependencies/gettext-runtime-0.17-1.zip")
+set (pkg_gtk_md5
+	"09baff956ebd1c391c7f71e9bd768edd"
+	"110394b4b1e0a50cd440f1e8729d159c")
 
 # https://sourceforge.net/projects/urlget/files
 # /GTK%2B%203%20binary%20for%20Windows/GTK%2B%203.16.6/