GPencil Tools: Add viewport timeline scrub feature

Add timeline scrubbing from the standalone add-on : https://github.com/Pullusb/viewport_timeline_scrub
This bind a shortcut to move in timeline directly from viewport, VSE and movie clip editors.
Display keyframes and snap using `Ctrl` key.
Propagate the chosen shortcut to timeline editors.
Shortcut, behavior and overlays can be customized in addon-preferences.