Fix button to Qt::MouseButton mapping

Even though the names seem to match, QtWayland maps button values
to enum values in ascendung order (as it does on X11). The wrong
mapping is usually not a problem because we send the native button
events to clients. However when the Qt names or values are used
for communication between KWin and a client this leads to
misunderstandings.
BUG:465463
FIXED-IN:5.27.1


(cherry picked from commit cc4d99ae)