xwayland: Fix sending multiple XdndDrop messages after a drop occurs
There might be more XdndStatus client messages after a drop occurs. In which case avoid sending XdndDrop client message again as it can result in a feedback loop with clients such as GIMP.
(cherry picked from commit 48faf831)
Co-authored-by: Vlad Zahorodnii vlad.zahorodnii@kde.org