Screen size detection for gui-gtk3.22+
Gtk deprecated the GdkScreen APIs we were using to gauge screen size, so we removed them from gui-gtk3.22 in gna patch #7877 . This left the Gtk3.22 client without a way to adapt to screen size.
It's possible to get a size from the GdkMonitor API to get the same sort of information. It's documented as new in Gtk3.22.
(I'm using gdk_display_get_primary_monitor() on the default display, which might not be where the user has launched Freeciv; but it's probably called too early for the alternative, gdk_display_get_monitor_at_window() , to be sensible. This is mostly about sensible defaults, anyway, so it's not critical to get the right monitor; only
low_citydlg can't be overridden by the user.)