Only save state if emulator supports it.
We need to only support bookmarks if the underlying emulation engine properly supports it. This will solve a lot of problems when people try to load save states and emulations like MESS, while they support save states, the system selected under MESS might not (most of them!).
This adds a boolean to the emulation model, an accessor in the controller, and some logic to save state only if the subclass sets the appropriate boolean.