option for setting surround sound / channel layout

Simple as the title says. The sound preferences in elementaryOS do not have an option for setting channel layout (stereo, quad, 5.1, 7.1, etc). I understand that most sound output setups are stereo, and that elementary's philosophy is generally KISS, but this seems like a major oversight for anyone who actually has a multichannel sound setup.

Note that once the sound was correctly configured (using pavucontrol), the sound preferences can actually do a 7.1 channel speaker test, despite there being no option to select this configuration via the sound preferences.

No file or test is named. Start by locating the sound preferences and the existing 7.1 speaker-test behavior in elementary/settings-sound; done means users can select channel layouts such as stereo, quad, 5.1, and 7.1 from the sound preferences.

Written by the indexing model from the issue text.