Blender Git Loki

NLA SoC: Transition Strips + Strip Adding Operators + Bugfixes

== Transitions ==
Transition strips are now able to be created + evaluated. Transitions allow for interpolation between the endpoints of two adjacent strips in the same track (i.e. two strips which occur in the same track one after the other, but with a gap between them).

- The current behaviour when only one endpoint affects some setting is non-optimal, since it appears somewhat inconsistently extend/replace values...
- Transform code needs a few fixes still to deal with these


== Strip Adding Operators ==
* New strips referencing Actions can be added using the Shift-A hotkey while in the strips-area. You must have a track selected first though.
The new strip will get added, starting from the current frame, in the selected track(s) only if there is enough space to do so. Otherwise, the new strip gets added at the top of the stack in a new track.

* New transition strips can be added with the Shift-T hotkey while in the strips area. You must have two adjacent strips selected for this to work.

== New Backend Methods ==
* Recoded the strip/track adding API to be more flexible
* Added a new method for testing whether F-Curve has any modifiers of with certain attributes. Will be used in a later bugfix...

== Bugfixes ==
- Fixed bug with strip-blending which caused the blending modes to be useless.
- NLA buttons now use proper poll callbacks instead of defining checks
- Commented out missing operator in menus, silencing warnings in console
- Removed obsolete/incorrect comments