Add defines for "time source"

Time source node + time source relationships are used to indicate
things which should be changed when frame changes (e.g. animation
needs to be run, certain modifiers should be updated, etc.)
Start of experimental code for building depsgraph

Currently there's nothing here except for some includes and an entrypoint
for the building process, starting from scene. Aim here is to start working
on implementing this stuff to clarify a bit design issues of main API and
datatypes, as any difficulties encountered during this process will end up
revealing problem areas in the design (as well as giving us a more solid
technical grounding when looking at things)