gtk4: Build against gtk4 (stable)
Turn gtk3x-client to gtk4-client by building against stable gtk4 (no longer gtk-3.9x.x development versions), and renaming variables etc from "gtk3x" theme to "gtk4" theme.
File windows/installer_cross/freeciv-gtk3x.cmd should be renamed.
freeciv-gtk3x.cmd renamed in latest versions.
Turn gtk3x-client to gtk4-client by building against stable gtk4 (no longer gtk-3.9x.x development versions), and renaming variables etc from "gtk3x" theme to "gtk4" theme.