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Add bone_groups.new() and bone_groups.remove() methods to RNA.

To do so, matching BKE 'API' was also refactored a bit:
* Get Pose data instead of Object, as parameter;
* Removed some sanity checks not needed at such a low level (callers are supposed to do that);
* You can now remove an arbitrary bone group, not only the active one.

Based on patch by pkrime (Paolo Acampora), with own edits.

Reviewers: #python, pkrime, aligorith

Reviewed By: aligorith

Differential Revision: https://developer.blender.org/D522