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

Migrate all users to using the InputModel.

Issue DMDirc/DMDirc#426

With the removal of message sinks, the active window plugin has
nothing to do now. We can reimplement it much more simply
directly as part of the Swing UI.

Start to move FrameContainers behind an iface so they can be
pulled out.

For now, just cast back when needed.

Closes #207