FR: Append to timeline

Single keystroke 'append' function (as an alternative to insert/overwrite/insert before/insert after/overlay).

Adds clip to end of timeline (obey track light / patching / source in + out) ignoring sequence in/out. 

Sequence to otherwise remain unaffected (i.e. sequence in/out, playhead position, track lights).

Bonus: being able to assign sequence to append destination - so it could be an already pancaked selects sequence.