Gecko:Layout Improvements

This started from User:Dbaron's list, and some probably belong in Gecko:Content Team Minutes instead.

A. Reflow refactoring

B. Global object proxying so that one global object per document -- maybe proxying for frame.window rather than subitituting during load?

C. Stylesheet loading not blocking parser.

D. Units rewrite

E. Make XML content sink incremental (maybe switch to libxml2 at same time)

F. Fix incremental page loading timer + paint supression nightmare (depends on (B) and (C)).

G. Adaptive zoom (depends on (D)).

H. Make Print Preview use its own window.

I. Focus mess - make it a tree

J. (related to (I)) - have a better notion, for embedding and for making tabbrowser less hacky -- of having a docshell hidden (no resize reflows, no focus, no gif animating, etc.)

K. Make hidden views mean something, perhaps?

L. Frame constructor refactoring and movement towards extensibility. Maybe put style contexts on content.

M. Docshell cleanup (related to (J)).

N. [Gecko:NativeWidgetOverhaul Move to one widget per top-level window] --- no child widgets.

O. Eliminate the view manager and the separate view tree; just work off the frame tree and move all the functionality to the presshell.