In preparation for child eventbusses being removed, anything
trying to subscribe to specific windows now needs an explicit
check.
In the process, fixed two issues:
- channel who plugin was sending way too many requests - each
connection was querying every channel in the client. i.e.,
with 5 servers open there'd be 5 WHOs per channel...
- ServerFrame stops itself listening when *any* frame is closed.
So closing a channel window then reconnecting wouldn't offer
an SSL dialog, because it's already unbound the listener.
Issue DMDirc/DMDirc#662