Styles cleanup

[danirabbit]

Could be rebase merged :shrug:

There's other things I want to do, but I feel like it would be better to just do some general code cleanup first

DisplayWindow:

• Make the selector more obvious for the DisplayWindow
• Apply alpha to the DisplayWindow background instead of opacity to the whole widget. Also keeps styles in one place
• GObject-style while we're here

DisplaysOverlay:

• Only construct the display css provider once instead of once per monitor