* The thumbnails are now loaded using a Gtk.FlowBox, which saves a lot
of code.
* Cheese's `coldplug()` is now done asynchronously, since it otherwise
blocks the UI for several seconds.
* Minor formatting changes.
* Add some comments to make things clearer.
Libgd is bound to remove Gd.Notification, and removing it also means it
gets easier to port our build system to Meson.
Furthermore, we can now use some more sane defaults for our application
w.r.t. notifications.
