dnd: Fix preferred action on x11 drops

I thought we had fixed setting the preferred action on a drop in x11 here gtk!4785 (merged)

Unfortunately, my testing didn't include the actual drop itself. It appears that everything was working because the drag was updated correctly (which controls the cursor icon).

There's work on a solution to set the preferred action on the GdkDrop, but this is unlikely to happen before code freeze. gtk!4982

After extensive discussion with Company on IRC, we haven't come up with a solution. The current proposed solution in gtk!4982 sets the GdkDrop action in drop_status which is not appropriate.

I feel that this is important for 43.rc given the many unexpected results it can have (i.e. #2457 (closed))