Multimonitor Support

[ibabushkin]

This would be a pretty big change. For now, determine what the needed changes to existing code and infrastructure are.

• [x] wrap a screen's root window (together with some info on location), a tagset, vector of visible windows and screen geometry into a struct and place a vector of those in the Wm struct. Clarification: we still only have one screen/root window. Also, visible windows might also have to stay in our Wm object, I'm not sure.
• [x] allow for focus changes across screens
  • [ ] implement neighbour relationships across screens
• [x] interact with RandR
• [x] don't hide windows when monitors disappear
• [x] implement better visualization of tagset overlap (as in, provide the information necessary)
• [ ] make redraws local to the affected screen(s)

[ibabushkin]

Regarding local redraws - this is a quite annoying change, and only brings marginal benefits. This is not to be expected to be added soon ;)