iZotope plugins will already send messages when connect() is called on
the first object, so this flag has to be set on both host contexts at
the same time.
This has much fewer moving parts, and it's probably more understandable.
There was also a race condition in the previous implementation, so
there's that.
This requires a super hacky workaround because the UI thread can be
currently blocked by the plugin calling `IPlugFrame::resizeView()` from
the Win32 message loop.
As it turns out there are only two or three functions where we can do
this. It also breaks logging, and this function will probably only be
called once anyways. More consistency is always better.
This is quite a huge refactor, but note everything is consistent (and
we're going to need one or two more of these `Vst3*Proxy` objects).
Right now nothing extends `IHostApplication`, but this way it will be
trivial to add support for more host context interfaces.
Robbert van der Helm