Blender Git Loki

- sequencer clipboard now stored globally (not in the scene, makes pasting into other scenes nicer)
- multiple pastes after copying
- clear the sound handle when copying (was crashing)
- allow seq freeing without a scene (assumes seq strip isnt active and sound handle isnt set)
- free clipboard sequences on exit
- paste sequence strips using the relative playhead location from when they were copied.

TODO
- check scene pointers on paste
- detect overlaps after paste