Keyframe between Camera (.vc3) files

[sgpearse]

Keyframing can be done with the PythonAPI by providing a list of session files. As we move forward, keyframing between session files might entail more than just moving the camera. It would be helpful to keyframe between just camera files (.vc3) without including all other parameters that go into a session.

.vc3 files can be created in the Scene->Viewpoint tab under the Camera Controls section.

[sgpearse]

Related to #3317, #2230