Sequencer usability:

Brought back old tools "Remove Gap(s)" and "Insert Gap".
It's actually one of the first tools I ever coded for it in 90ies, so useful!

* Remove Gap(s)

This checks if there's no strip at a given position, and slides all strips
together to the left, until the gap is closed.

- BackSpace key, remove gap at current frame (or first gap at right of frame)
- SHIFT+BackSpace, remove all gaps at or to right of current frame.

* Insert Gap

Shifts all strips to right of current frame with 10 frames. (Amount can be
set in Toolbar redo panel).




git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@55600 954f8c5b-7b00-dc11-b283-0030488c597c