Notifications rework

Initial implementation of a app wide notification system.

TODO:

• This make all those GtkOverlay and GtkRevealers redundant, I have still to remove them from the UI files.

Depends on: !292 (merged)

Closes: https://gitlab.gnome.org/World/PasswordSafe/-/issues/277

Fixes: https://gitlab.gnome.org/World/PasswordSafe/-/issues/120