wayland: Prevent matching dnd actions after drop

Currently, kwin will continue matching dnd actions after a drop. This is
mainly needed in order to make "ask" action work.

However, it appears like there are clients in the wild that update offer
actions after receiving wl_data_source.dnd_drop_performed and wl_data_device.drop
events.

One could argue that's a client bug, and perhaps it is, but the spec is
vague about that. In meanwhile, let's make sure that dnd action matching
is active after drop only when the selected data source action is "ask."