This should really be split in separate semantics for key and button events.
Fixes a subtle but nasty bug in the EngineDialog where the change of the default focus
widget from a Gtk::Button (acts on press) to an ArdourButton (acts on release) caused
events occuring after a dialog has grabbed focus to trigger button clicks
When there's no main window (initial setup, no transient parent),
preset a normal window listed in the task-bar.
The duality the Engine Dialog being used as Ardour-WM managed non-modal
Window (Menu > A/M Setup) and modal Dialog (AudioEngineSetupRequired)
complicates this a bit.
OK -> start_engine() can eg. trigger an interactive plugin scan,
which in turn leaves the EngineDialog responsive. changing settings
or clicking OK again can lead to undefined behavior.
It may happen that during push_state_to_backend() a device is
reconfigured in a way that triggers a "Device Changed" callback before
the engine is started. This callback can trigger a change to the
configuration that will be used when the engine is actually started.
This has been seen on OSX in conjunction with Aggregate Devices
(even if the aggregate is not used, but the device which is used
is also part of an aggregate)
example: HW changed callback arrives, device-list is re-populated,
*A*irplay" is at the top of the list, Airplay supports only 44.1K,
Samplerate changes... later save also writes this new rate to the file.
This replaces using ARDOUR_UI::disconnect_from_session which is only used by the
EngineControl class. ARDOUR_UI::disconnect_from_session also disconnects from
the AudioEngine::Halted signal which seems unnecessary as Halted is not emitted
when stopping the engine and calling update_sample_rate() which is already
handled when the AudioEngine::Stopped signal is emitted.
This has to be handled in two places, in ARDOUR_UI::do_audio_midi_setup and in
the dialogs response handler and in as the window can also be triggered via the
window action manager.
