Blender Git Loki

Depsgraph: initial move towards switch to the new depsgraph

Quite some changes in this commit, mainly:

- Made it more clear separation between old and new dependency graphs.

So now logic for the new dependency graph is totally concentrated in
depsgraph/ folder and old DAG_* calls only redirects to that module.

This gives a bit of code redundancy (not so much tho) but it makes
it really clear how to drop old dependency graph or how to make it
an easy switch between old/new code bases (as a command line argument
perhaps to be safe for a major failure).

- Kind of part of previous one, but to be clear: all the parts of the
new dependency graph API which were marked as TODO are now filled
in.

It doesn't mean they're 100% finished, but initial code is there.

- Removed some experimental code. Let's just stick to code which might
not be optimal but works reliably.

- Started re-integration new dependency graph into editors.

This is basically Cycles viewport rendering which starts working
now. Still some glitches in there and missing updates but let's
start from something.

I decided to not invent anything totally new here and just ported
old logic with id->flag tags.