MPRIS2: Get the icon name from the player application
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.