This is also something that we did upstream. Since we want to
introduce an explicit "xdg_transient" window type for tooltips
and popovers, and since "transient_for" is a confusing dumb
80s term lifted from the ICCCM spec, just rename it.
This was changed upstream a little while ago for C++ compatibility.
It's also the more common term for the operation: you close a window,
you don't delete one. In fact, a delete event might seem like it
would be about resource management instead.
