Plug windows weren't redrawing properly because the embedded
window was expecting to get messages for each frame from the
compositor, but the compositor doesn't know about embedded
windows. Simply disable frame sync for GtkPlug's GdkWindow -
extending XEMBED to handle frame sync isn't interesting
at this point.

A new API gdk_x11_window_set_frame_sync_enabled() is added
to allow this to be done.

https://bugzilla.gnome.org/show_bug.cgi?id=701613