Multiple Monitor Enhancements

[koraa]

• Fix bug where a wrong geometry is set for multiple monitors
• Fix bug where the geometry is not being updated when a monitor is added removed

[hamaxx]

I reverted some of the changes from this commit because they broke the layout for me(everything on the primary screen got squeezed to one pixel).

https://github.com/hamaxx/unity-2d-for-xmonad/commit/b2253a435566eb2d22de467657a0f0689be1b5e3