Add ability to hide non selected indicators in preview

[Krealle]

The indicator preview can get a bit noisy when you have multiple indicators enabled. I'm finding that I sometimes have to temporarily disable some of my indicators to be able setup other indicators.

So, in this PR I've added a checkbutton, in the preview section, where you can toggle between showing all active indicators, and only showing the currently selected one.

[enderneko]

Finally got some time to do this. Thank you!