Sound settings design updates

I've come up with some updated designs for the Sound settings. They aim to resolve the main complaints that we get about the sound settings, and should theoretically resolve #501 (closed), #817, #1305 (closed), #1762 (closed), #1584 (closed), #1910 (closed), #2033.

The main changes:

• Move the app volume levels into a secondary window
• Use standard AdwComboRows for device selection
• Remove the "system volume" slider, and move it into the device section, as "Output Volume"
• Move alert sound selection into a secondary window