The geometry widget feature of gtk_window_set_geometry_hints() has
never really worked right because the calculation that GTK+ did to
compute the base size of the window only worked when the geometry
widget had a larger minimum size than anything else in the window.

Setup:
* Move the GtkSizeGroup private functions to a new private header
  gtksizegroup-private.h
* Add the possibilty to pass flags to _gtk_size_group_queue_resize(),
  with the flag GTK_QUEUE_RESIZE_INVALIDATE_ONLY to suppress adding
  the widget's toplevel to the resize queue.
* _gtk_container_resize_invalidate() is added to implement that feature
* _gtk_widget_override_size_request()/_gtk_widget_restore_size_request()
  allow temporarily forcing a large minimum size on the geometry
  widget without creating resize loops.

GtkWindow:
* Compute the extra width/height around the geometry widget
  correctly; print a warning if the computation fails.
* Always make the minimum size at least the natural minimu...