Add keyboard shortcuts for markers management
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.
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".