Consider ability to add custom actions

[sea-bass]

This would be a big change, but potentially a good one. What this would require is:

1. Register (and specify) new actions in the Robot class, as well as in the Action class!
2. Configure which action buttons show up in the GUI, and dynamically create button callbacks/runners.
3. Create new PDDL domains with these new actions. This includes the PDDL to action conversions.