Blender Git Loki

Sequencer: various preview selection improvements

- Only cycle items when the cursor hasn't moved.

This matches object-mode behavior, making it possible to tweak-drag
the current selection without first cycling to the next sequence strip.
Successive clicks will still cycle sequence strips.

- For center selection, use a penalty for the active strip.

- Use a temporary selection list to avoid moving the sequence
strips out of the scene during selection
(changing their order when added back).