Remove Stylist-Traits APIs from Stylist Core, leaving backwards-compatible stubs.

Add new style tree traversals.

Implement style tree flattening utility.

This will be useful in Rhapsode for making sure voice synthesizers continue to
have full voice features information as the user jumps around the document.
And it will be useful for visual browsers in transitioning from layout to render.

Abstract Tree traversal (commited out-of-order).