edge-resistance: Add snapping for tiled windows

When windows are tiled, it improves the interaction with
them when they have a set of snapping edges relative to
the monitor. For example, when there's a document editor
and a PDF file opened, I might want to rescale the former
to 2/3 of the screen and the latter to 1/3.

These snapping sections are not really tied to any other
window, and only depend on the current work area of the
window. Thus, it is not necessary to adapt the current
snapping edge detection algorithm.

This patch adds the necessary code in edge-resistance.c
to special-case tiled windows and allow them to cover
1/4, 1/3 and 1/2 (horizontally) of the screen. These
values are hardcoded.

https://bugzilla.gnome.org/show_bug.cgi?id=645153