Add `AdwTabOverview` and remove some deprecated widgets

This pull request adds an AdwTabOverview (as per #339 and #265 (closed)) to Black Box, and in doing so also modernizes a few widgets in various places.

To-do before merging:

• Filter non-monospace fonts in the Font Selection dialog
• Regenerate translations

Screenshots:

Note: There are bugs with handling dragging tabs between empty windows, but these are upstream libadwaita bugs that I need to report. :)