Preferences: Add preview switch and placeholder
This adds a 'Show live preview' checkbutton to the AV page in Preferences. Instead of connecting to the stack page’s map event, this checkbutton is used to toggle the preview. Changing input devices works while the preview is enabled (changes setting and switches device), as well as when it’s disabled (changes setting).
I also added a placeholder for the video preview and wrapped the whole page in a ScrolledWindow.