Improve function signatures of registerAction and registerActionCallbacks

[justvanrossum]

These currently take three positional arguments for three callbacks, the latter two being optional. It would be better if we pass a "callbacks" object, like this:

    registerActionCallbacks(actionIdentifier, { callback: ..., enabled: ..., title: ... });

This should be easier to remember than the argument order.

(This will have to wait until after #1925 is done.)