Use overlay to show empty message

[jeremypw]

Fixes #1224

Use an overlay in preference to using custom drawing or a stack so that user input and DnD still goes to view (except if dropped onto the empty label).

Custom drawing could probably still be done in Gtk4 using Snapshot and Paintable functions but I would prefer to avoid this if possible.

[zeebok]

Code looks good. I can give this some testing tomorrow when I get home if desired

[jeremypw]

@zeebok Thanks for the review! Practical testing by another person is always preferable if possible but there is no hurry.

[lenemter]

LGTM