Use a single function with the complete logic.
Since the callgraph is complex, there is internal state as well as GUI
state (different pages), do not rely on individual methods to get it
right.
A widget's sensitivity should only be controlled by one function.
This can happen if both input and output devices are set to None for instance
on backends that report no supported SR or buffer size for the None device.
EngineControl::set_driver_popdown_strings is now like the other
set_*_popdown_strings methods in that it sets the driver strings and a
default active entry and returns false if no drivers are available.
The fix does not work properly anymore for the Portaudio backend and needs
fixing in another manner. It will now be temporarily back to the broken state
it was in before the "fix".
This stops a whole lot of redundant signal emission and makes it much easier to
think about what is going on. It also makes the dialog present much faster.
AudioBackend::enumerate_drivers is not supposed to be called for backends that
don't support driver selection. The wavesaudio backend asserts if the method is
called.
Connect to the backend_combo changed signal after setting state as calling
backend_combo.set_active_text() in set_state was triggering backend_changed(),
which would then see the driver_combo had not been set and set it to the
incorrect value.
The value/name of the backend needs to be restored first then we can populate
the driver combo and set the correct active entry from the saved state. After
which backend_changed() will populate the device combo's etc so they can then
be set to the correct active values from the saved state.
Some refactoring was necessary to avoid code duplication
Restoring of device state for input and output devices still doesn't work
correctly. I'm not quite sure what the issue is at this stage.
For some reason we don't understand 'push_state_to_backend()' can interfere with hiding the dialog (causing a partially hidden dialog to remain on screen). It's most likely a timing issue with the Windows version of GTK. Fortunately, reversing the calling order seems to fix it - BUT...
If there's no session loaded yet, the user can be left with a very long wait while nothing seems to be happening. The next thing the user would normally see would be the splash image. So let's display it slightly early (so the user can at least see that something's happening).
Sadly, it's all very kludgy - but a lot better than what we had before... :)
