Blender Git Loki

Action Constraint: Add manual time factor input control

Adds an optional slider to the action constraint so that it can be
driven without a constraint target.

This is very helpful for more complex rigging and mechanical rigs, as it
means the action constraint can be controlled with a driver/custom
property directly, currently if we want to use a driver to control it we
must add a "dummy" bone/object inbetween to act as a control.

Reviewed By: Sebastian Parborg, Sybren A. St�vel, Demeter Dzadik, Julian Eisel

Differential Revision: https://developer.blender.org/D8022