The plugin's UI factory creates and displays dialogs from an arbitrary thread.
It needs some utilities to invoke certain methods, wait for their return, and pass
information back to the calling thread. This should at least be made robust and
general enough for the plugin to use for its use cases, but may be useful for
other applications as well. This somewhat alleviates the need for "modal dialogs",
but there may still be a need for that in the "mixed code" scenario.