Started refactoring lyaout.

To make it faster. Currently all broken :/
Added unit test to test breakage.
Made test pass.
Use measure layout algorithm for arrange.
Don't remeasure/arrange when size hasn't changed.
More work on layout.

Mostly works but Test_ScrollViewer_With_TextBlock is failing again.
Fixed a few more layout bugs.
More stylecop fixes.
Fixed final layout problem i'm aware of.

(Currently!)