libevent: switch to embedded data pattern.

libevent: switch to embedded data pattern.

Note that previously pane_register was being called with a NULL parent,
though this was hidden from smatch.
This meant that pane_damaged() skipped this event:idle registration.
Fixing that resulted in extreme recursion.  That wasn't really
needed for libevent, so break it.

Signed-off-by: NeilBrown <neil@brown.name>