move control app launching back into audio backend to allow ASIO/CoreAudio model to work; push initial state of AMS dialog to backend

Issues remain with the basic model of the AMS dialog - when is
newly chosen state pushed into the backend (which can then
modify the control app button sensitivity. This is a special
problem for this button because APIs like ASIO and CoreAudio
probably don't allow us to launch a control app for an arbitrary
device, but only one actually in use. In this sense it is
different from properties like available buffer size etc, where
we can typically query without actually using the device.