Improve RelativeLayout by allowing dependencies to be declared in a random

order.

The new implementation uses a dually topologically sorted graph of the child
views. The graph of dependencies is sorted once for the rules that impact the
horizontal axis (toLeftOf, alignRight, etc.) and once for the rules that impact
the vertical axis (above, below, etc.)

Doing so gives the ability to declare dependencies in any order, allows for
partial cycles in the graph (given view1 and view2, view1 can be toRightOf=view2
and view2 can be above=view1) and probably gets rid of most surprising behaviors
of RelativeLayout.