Bugfix and side-by-side tiling

Nice to hear that metacity finally gets some attention :). I've been running a modified version of metacity for months. I've made two changes:
- An alternative monitor edge algoritm to fix this bug: https://bugzilla.gnome.org/show_bug.cgi?id=684212
- side-by-side tiling, also know as 'Aero snap' (mostly backported from Mutter, but I made a few changes to make it work in metacity and fixed another bug).

Your master branch is broken now because of the gtk thing, so I only tested it against the commit right before the one that broke it (look at my old-gtk branch).