Wrap GtkOverlay widget

	* Create gtk/src/overlay.{hg,ccg}
	* Update gtk/src/filelist.am
	* Add "get-child-position" signal to gtk/src/gtk_signals.defs
	* Add GTK_TYPE_OVERLAY to
	tools/extra_defs_gen/generate_defs_gtk.cc
	* Add overlay.h to gtkmm.h