GtkCssBoxes

Here's a set of my latest refactorings that I wanted to pass by @baedert for comments on his transforms work before merging, because I suspect it's going to cause a tiny few bit of conflicts.

The big changes for that are in commits d3a2218f (which splits priv->allocation) and 285aba6e (which makes priv->transform be the parent->child transform).

As a side effect, it's also introducing the GtkRoot patches from wip/otte/snapshot and elsewhere that multiple branches have wanted, but doesn't do anything that big with them yet.

Comments?