Add dexterity support

I've added dexterity support by changing the subscribers from AT-events to zope.lifecycleevents.

Included:
- tests verifying that AT objects still work and are triggered only once
- tests verifying that Dexterity objects work and are triggered only once
- workaround for breaking loop because we are subscribing to the modified event which is triggered when setting the coordinates.

@lukasgraf could you take a look?