use multiple cameras

[gperciva]

.actions files should have a way to switch between camera, i.e.
camera 7.25 2

• add to unit test
• make blender python scripts read these values and switch the active camera
• make sure that models have cameras named "camera-X" where X is an integer.

[gperciva]

There's currently a --cycle-cameras=X seconds, where it switches between all available cameras every X seconds. No functionality to control this manually yet, though.

I think this will really depend on adding it to the lilypond input, which will require me to get my hands dirty with scheme stuff and lilypond internals again. (I need either a custom event, or a extra-event which allows me to add whatever list(s) I want, or something)