Add keyboard shortcuts for markers management

Currently MarkerBox.do_button_press_event takes care of the interactions with markers. The logic which creates/removes/edits a marker should be refactored into actions which can be assigned keyboard shortcuts. https://gitlab.gnome.org/GNOME/pitivi/-/blob/c96cedb4608b3aa0528ef27a3937498b83b2fb0d/pitivi/timeline/markers.py#L196

The "add" operation already has a keyboard shortcut, introduced in c96cedb4

Still to be done:

• "remove" once #2454 (closed) is done
• "select a marker for moving", which would then allow the marker to be cut and pasted when using "add".