Fix: clients/window: Premature finish request when copy-pasting

As per the wl_data_offer::finish documentation, the request is only
valid for drag n drop operations and signifies that a dnd is completed.
Send finish request only when we have a dnd operation active.

Signed-off-by: Harish Krupo <harishkrupo@gmail.com>
(cherry picked from commit 43152a3a)