Emit DataOfferInterface::dragAndDropActionsChanged() only if dnd actions change

With some apps, firefox specifically, it's possible to enter a dnd
positive feedback loop, e.g.

* kwin sends wl_data_offer::action event
* the app reacts to it and calls wl_data_offer::set_actions() with the
  same actions
* kwin re-sends the wl_data_offer::action event because
  DataOfferInterface::dragAndDropActionsChanged() has been emitted
* the app calls wl_data_offer::set_actions() again, and so on...

This should make D&D in Firefox more responsive.