Add empty methods to screen class, for #353.
Add empty methods to screen class, for #353.

Gracefully ignores features that don't make sense in live coding mode.

brandm/live-py-plugin,donkirkby/live-py-plugin,brandm/live-py-plugin,brandm/live-py-plugin,donkirkby/live-py-plugin,donkirkby/live-py-plugin,donkirkby/live-py-plugin,donkirkby/live-py-plugin,brandm/live-py-plugin,brandm/live-py-plugin