Clean up the backend filter and multi driver settings

Fully filter the backends in MixerToolBox if a filter is set,
for hotplugging as well as initial probing.

Resolve the backend filter list and the multi driver mode in KMixApp,
taking values from command line arguments (if provided) or from hidden
settings (if present).  Pass those directly to MixerToolBox without
having to go through member variables in KMixWindow.  This greatly
simplifies the parameters of MixerToolBox::initMixer(), which were
effectively application-global anyway.

Accept informational command line arguments ("-help", "--version"
etc) even if KMix is already running.