Previously there was a hardcoded list of applications in
Mixer_MPRIS2::getChannelTypeFromPlayerId() which passed special
MixDevice::ChannelType values to MixDevice::channelTypeToIconName().
Now the icon name is found via the application's desktop file, which
is requested from the running application via DBus, and passed directly
to MixDevice.  Icons for audio devices are still managed by MixDevice.

There are some applications which do not provide the DBus interface
to locate their desktop file.  Therefore, there is still a hardcoded
fallback list in Mixer_MPRIS2::getIconNameFromPlayerId() which provides
an icon name.  The MixDevice::ChannelType values for specific
applications are no longer used.