[klipper] Workaround race when klipper updates on wayland

We have a situation where some clients drop their old offer before
creating a new one. This means klipper tries to fill in the empty
clipboard at the same time the client posts its new real contents.

This adds in a flag (via a hidden mimetype) that klipper is trying to
replace a null clipboard. If this flag is set and our clipboard is not
null because the client has updated it in the meantime we ignore the
klipper update.

It's a workaround, rather than an ideal fix at a data level, but it
solves the problem in the interim.