Blender Git Loki

NLA: Shift-Tab toggles tweakmode AND makes that track solo

To help make it more convenient to edit stashed actions, Shift-Tab
(i.e. holding down the Shift key, which "tabbing" into tweakmode as
usual to edit the action referenced by the active NLA strip) now flags
the NLA Track that the strip occupies as being "solo" too.

This allows you to use the NLA to select a stashed action, then Shift-Tab
to start editing it without any other actions in the NLA stack interfering.
Like the "Next/Previous Layer" tools in the Action Editor, this is designed
to help with checking on stashed actions.