Blender Git Loki

Implement grouped undo option for operators

This option makes an operator to not push a task to the undo stack if the previous stored elemen is the same operator or part of the same undo group.

The main usage is for animation, so you can change frames to inspect the
poses, and revert the previous pose without having to roll back tons of
"change frame" operator, or even see the undo stack full.

This complements rB13ee9b8e
Design with help by Sergey Sharybin.

Reviewers: sergey, mont29

Reviewed By: mont29, sergey

Subscribers: pyc0d3r, hjalti, Severin, lowercase, brecht, monio, aligorith, hadrien, jbakker

Differential Revision: https://developer.blender.org/D2330