GtkWindow: Allow setting size from geometry

GtkWindow: Allow setting size from geometry

If you set a geometry widget via gtk_window_set_geometry_hints() it
becomes very hard to compute appropriate toplevel sizes in pixels
to make the window a particular size. Synthesizing strings and passing
them to gtk_window_parse_geometry() is possible, but to avoid
avoid such ugliness, add functions:

 gtk_window_set_default_geometry()
 gtk_window_resize_to_geometry()

That act like gtk_window_set_default_size() and
gtk_window_resize() but are in terms of the resize increments of the
geometry widget.

https://bugzilla.gnome.org/show_bug.cgi?id=631796