NLA Editor: Swap Strips (Alt-F) and Bugfixes

1) Added a new operator to swap the order of strips within a track
(Alt-F).
This makes it possible to select two strips (or more precisely, two
islands of consecutive + selected strips) in a single track and change
the order in which the appear without needing a extra tracks to
perform the move through.

As usual, the non-overlapping rules apply, so there may be some cases
where swapping in this way is not possible without adjusting the
intermediate strips first manually. Otherwise, everything just gets
too tricky to manage deciding what adjustments should be done to the
obstructing strips to make a fit.

2) Freeing meta-strips didn't free their local data properly (i.e.
modifiers they may have had).

3) Adding strips to tracks, where the endframes for the strips
overlapped would cause problems with incorrect ordering of strips. I
still need to double-check whether evaluation works ok in this case...

release/scripts/ui/space_nla.py

@@ -139,6 +139,7 @@ class NLA_MT_edit(bpy.types.Menu):
         layout.operator("nla.clear_scale")
 
         layout.separator()
+        layout.operator("nla.swap")
         layout.operator("nla.move_up")
         layout.operator("nla.move_down")