move engine dialog from a widget to an actual dialog; emit Session::AudioEngineSetupRequired when loading a session if it is necessary
add can_change_{sample_rate,buffer_size}_while_running() methods to an AudioBackend

Allows the GUI and other stuff to know whether or not changing the SR/bufsize is possible while running, which is about to become useful
initial work on adding access to h/w control apps when ardour starts JACK