Warn when calling gtk_window_parse_geometry() on an empty window

Warn when calling gtk_window_parse_geometry() on an empty window

gtk_window_parse_geometry() gets the size of the window in order
to interpret the position of the window; calling it before, say,
calling gtk_widget_show_all() on a window is a subtle trap, so
add a warning in the case we can easily detect.

https://bugzilla.gnome.org/show_bug.cgi?id=631794