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)).