X11 positioning support

[ErikReider]

• [x] Positioning
• [ ] Don't overlap with Polybar (if even possible?)
• [ ] Get keyboard shortcuts working
• [ ] Try to use WindowType TOPLEVEL instead of POPUP
• [x] Get background opacity working
• [ ] Update README with text that states that this is experimental
• [ ] Print to stdout when started that X11 support is experimental
• [ ] Make sure that margins work
• [ ] Make sure that all positions work
• [ ] Make sure that all height and width config options work

[berbiche]

Don't overlap with Polybar (if even possible?)

I don't know whether EWMH would allow you to position the widget below polybar automatically, but an alternative is to set a top margin equal to polybar's height.