Fix deadlocks in wayland clipboard that can occur in special scenarios.

Fix deadlocks in wayland clipboard that can occur in special scenarios.

setMimeData() emits the changed signal always so to prevent duplicated
signals keyboardFocus() must only emit when the change came from
another wayland client. However direct connection may cause issues
when invoking the slot from a wayland callback, so use a metacall
to make sure we return from the callback. Unnecessary data transfer
and potential deadlock is now also avoided when a client is requesting
the mime data from itself.

Reviewed-by: Jørgen Lind