wlr_drag installs grabs for the full duration of the drag, leading to
the drag target not being focused when the drag ends. This leads to
unexpected focus behavior, especially for the keyboard which requires
toggling focus away and back to set.
We can only fix the focus once the grabs are released, so refocus the
seat when the wlr_drag destroy event is received.
Closes: https://github.com/swaywm/sway/issues/5116