ClapperGtkVideoadd_overlay

Add another GtkWidget to be overlaid on top of video.

The position at which widget is placed is determined from GtkWidget:halign and GtkWidget:valign properties.

This function will overlay widget as-is meaning that widget is responsible for managing its own visablity if needed. If you want to add a GtkWidget that will reveal and fade itself automatically when interacting with video (e.g. controls panel) you can use clapper_gtk_video_add_fading_overlay() function for convenience.