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. Stüvel, Demeter Dzadik, Julian Eisel
Differential Revision: http://developer.blender.org/D8022
c9c0f89307