Reviews and operations on the backend that support undoing can now be
committed immediately, so they will not be lost in the event of a crash.
This required tweaks to a few places:
- don't set collection mtime on save() unless changes were made in
Python, as otherwise we end up accidentally clearing the backend undo
queue
- autosave() is now run on every reset()
- garbage collection now runs in a timer, instead of relying on
autosave() to be run periodically
The type hints allow mypy to check the gui_hook calls, revealing a
bunch of places that are broken as they expect no arguments like the
legacy hooks.
To make mypy happy about PyQt's signal.connect(func), a qconnect()
helper has been added.
