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.
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