This is primarily to make the code reusable in other modules.  This results in some improvements:
* Modifier keys can be ignored.  For instance, ctrl-left drag defaults to rotation, shift must not be pressed, but the state of alt and meta is irrelevant.
* You could add an action that requires two mouse buttons.

This is a breaking change.

Conflicts:
	src/mapInteractor.js

Trigger more events to allow other interactors more fine-grained control.  For actions with a selection rectangle, use a flag in the action definition rather than a test against the action name.  Return the action record from the eventMatch function.  Add convenience functions for adding and removing actions.

The term 'event' was overused.

As part of this, there doesn't need to be a one-to-one correspondence between actions and events.  Moreover, selection rectangle-based actions can trigger any event.