Blender Git Loki

Recode of the "Set Scene chaining" patch from Nathan.

- code for detecting loops was overly complex, and detected loops wrong
even (when chains partially overlap it gave error)
- that code could NULLify sets of other scenes than the one you tested
- and that code should be in blenkernel, so it can be called in more
locations, like fixing potential loops in old files

- the code for looping over the chained scenes was bad too, I created a
nice #define to keep code readable. (See SETLOOPER in BKE_scene.h)

ALso fixes reports in tracker :)