Blender Git Loki

NLA SoC: More work on Meta-Strips

* Added several API functions for Meta-Strips editing. One of these (flush transform) still needs a few tweaks before it does its job well enough for all cases.

* Meta strips are now drawn with a purple-ish colour. The start/end points of the strips they encompass are shown with lines along the length of the strips, with lines starting from the top indicating start-points and lines starting from the bottom indicating end-points.

* Meta strips can be made (i.e. strips can be assigned to meta-strips) by selecting some strips and pressing Shift-G.
Meta strips can be removed by selecting some meta-strips and pressing Alt-G.

* Strips can now be 'snapped' to start from: the current frame, the nearest frame, the nearest second, or the nearest marker; using the Shift-S hotkey.
'Islands' of adjacent selected strips occurring in the same track are moved together as a single strip so that the start-point of the first strip is on the sepcified time, but all the relative lengths of strips stay the same. Internally, temporary meta-strips are created to facilitate this.