BETA: Added setting options to change opacity of background

Added 11 buttons under a 'Background Opacity' setting group to change the opacity of the background.

This isn't exactly finished, it works but with the current method of changing, it isn't very good, I couldn't figure out how to change the color of the button depending if it was selected or not, I tried to use the code from the Torrent Quality selection however nothing I tried seemed to work.

Also the 'Current Opacity' label doesn't show what level of opacity is actually selected and displays a letter which is what the settings are saved as due to CSS limitations.

Obviously some work is required on this such as making the active opacity a different color (ideally make them red by default and the selected one green) and make the 'Current Opacity' label display an actual percentage instead of the lettered variable.