GTK: stop using gtk_window_parse_geometry.

GTK: stop using gtk_window_parse_geometry.

This function, which parses the X11-style '-geometry WxH+X+Y' option
argument and automatically loads the result into the window, is also
being deprecated.

Fortunately we already had a fallback option for GTK1 (which didn't
have gtk_window_parse_geometry in the first place), calling the Xlib
geometry-parsing function and manually loading the results into GTK.
The method of loading into GTK is not the same between the two
versions, but the basic strategy is still viable.

For the sake of maintaining and testing fewer ifdef branches, I've
removed the use of gtk_window_parse_geometry _completely_, even in
GTK2 which did have it. GTK2 now uses the same strategy that I've
switched to for GTK3.