Using the currently active movie clip had different names in the interface:
In scene context it was "Active Movie Clip", in Camera Background Images it was "Camera Clip", in Constraints it was "ActiveClip".
I made all those instances use "Active Clip", which is descriptive enough and also the shortest of the three.
Reviewed By: billreynish
Differential Revision: https://developer.blender.org/D5400
17baa43e59