Fix client freeze due to deadlock (#1167)

See issue for deadlock cause. Removed need for ClientUser::get
call causing the c_qrwlUsers read lock in audio output thread
by moving feature implementation into GUI thread analog to how
application attenuation is handled. Now a global flag signals to
the audio output thread if priority speaker should be triggered
even though none of the outputs are priority speaker.

Also fixed asymmetric behavior between local and remote priority
speaker handling in the feature. Now also whisper and shouts trigger
the local priority speaker feature.

Minor refactoring in places that had to be touched to improve
readability. Removed some now superfluous comments.