Issue 284 fewer animation states

I've just had a bit of a poke-around and a play and I think that the `LOADING` and `REMOVED` states are both redundant now. The reasons for this are:
- The animation controls panel is only shown on animatable layers (no layers, no controls)
- The animation controls panel is greyed-out while animations are loading (so each control doesn't need to respond to the 'loading' state)