This is a function working only on a configuration which runs the
snapping with infinite snapping. This allows forcing a monitor that has
been modified to be adjacent to at least one monitor.
This commits adds a new arrangement widget, refactoring the existing
code and addressing a number of issues:
* Forced snapping made laying out more than 2 monitors hard
* Random gaps would be shown between monitors
* The scaling was fixed and usually tiny
https://bugzilla.gnome.org/show_bug.cgi?id=786971
