Adding support for window padding

Copied from other discussion.

> The bug where Amethyst won't resize windows a small amount is still present in this version.

> Steps to reproduce, launch Amethyst with a window padding of 0, layout a normal tall layout with 3 windows. Switch window padding to 10 and relaunch amethyst. Expected, windows are laid out with padding. Actual, windows shifted but not properly sized. Workaround, switch between two layouts


> There also seems to be an issue with moving a window from a larger screen to a smaller screen and it not being resized properly.