The goal is to make it possible to access evaluated datablocks at a
corresponding context. For example, be able to check evaluated state
if an object used for rendering.
Allows to write scripts in a safe manner for T63548 and T60094.
Reviewers: brecht
Differential Revision: https://developer.blender.org/D5726
Preparing for the bigger changes which will be related on passing
dependency graph to various callbacks which need it.
Differential Revision: https://developer.blender.org/D5725
