fixed intermittent rendering interruptions during live scene edition, and several refactorings.

- fixed intermittent rendering interruptions during live scene edition.
- introduced foundation::IAbortSwitch to implement custom abort switches.
- renderer::DefaultRendererController doesn't do anything anymore, and no longer has to be explicitly called by derived classes.
- throttling of the main render loop is now done by renderer::MasterRenderer.
- decoupled on_progress() and get_status() in renderer::IRendererController.
- the status value stored in appleseed::studio::QtRendererController is now accessed atomically.