ALSA: use non-modal error dialogs

ALSA: use non-modal error dialogs

aout_Restart() is quite buggy (IMHO) and restarts the audio output
from the interface thread. Then Qt4 external dialogs are buggy as
they deadlock if called from the interface thread (I cannot find a
direct way to use BlockingQueuedConnection for foreign threads and
DirectConnection for the interface thread).

Until either or both of those bugs are fixed, we cannot use modal
dialog from the audio output.