Blender Git Loki

'users of ID' py API.

This mainly adds bpy.data.user_map() method, which goes over the whole Main database
to build a mapping (dict) {ID: {users_of_that_ID}}.

Very handy to check and debug ID usages, but could also be really valuable for py addons
creating temporary scenes, or some exporters, etc.

Note: current code in master's libquery misses some IDs (and reports some it should not,
like nodetrees), this is fixed in id-remap but still needs serious review before going to master.
This basically means that current bpy.data.user_map() **will not** report a complete and exhaustive
state of dependencies between IDs. Should work OK in most cases though.

Original work/idea comes from id-remap branch, was heavily reworked by @campbellbarton
and myself for master.

Reviewers: campbellbarton, sergey

Differential Revision: https://developer.blender.org/D1678