Add API for out-of-process clients

[special]

QBackendProcess, Crimson.QBackend.Connection, and any other artifacts of the QML API from before instantiable types (and connection at import time) don't work anymore, and can't work. The connection still fully supports clients/backends in separate processes; it doesn't care at all.

Those should be removed and cleaned up, and we should figure out other API for QML applications that aren't launched by Go / backend/qmlscene. The QML plugin currently gets connection info from a context property, commandline argument, or environment variable.

One thought is that it's possible to configure/setup the backend from QML as long as that happens before the main Crimson.QBackend import is ever used. That's not too hard with the use of a Loader from the root object..

There should also be better/documented Go-side API for this.