Adds a sound mode selector

[glebsterx]

Adds a sound_mode list that will allow these modes to be controlled on audio systems and audio receivers until the media player is put into standby or turned off. Made by analogy with source_list, but with one caveat: by default, the GUI selector "Sound mode" does not disappear when the media player is turned off, so this feature is removed from supported_features every time it is turned off (because it is illogical to control the sound mode of a turned off device)

[stale[bot]]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.