Add another Widget to be overlaid on top of video.
The position at which widget is placed is determined from [property@Gtk.Widget:halign] and [property@Gtk.Widget: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 Widget that will reveal and fade itself automatically when
interacting with this (e.g. controls panel) you can use
add_fading_overlay function for convenience.
| this |
a Video |
| widget |
a Widget |