Fixes #60.

This sets up a global notification for changed textures. All views observe this notification and redraw themselves when the texture has changed. This is not exactly the cleanest possible way to set this up, but it works okay. In the long term, the texture probably needs some way to inform its users of any changes (in particular renames), which could then be used for the redraw notification.
Fixes #59. At least I hope. It's hard to reproduce.

I was already handling app deactivated events, I just didn't send the right mask. So for future reference: It's NSAppKitDefined.
Fixes #61
Checks for locked cameras now happen only where the camera is to be updated, but they happen consistently there.