dom/ and content/ source reorganization

In order to make our codebase more accessible, I'm planning on doing some
reorganizations of source files. Here is my plan for content/ and dom/. jst
has looked it over, but I'd like to post it publicly before beginning to
implement it:

1) put everything under dom/

2) Keep the public IDL interfaces in roughly their current form in
dom/interfaces

3) Use the following other directories:

dom/base (nsIDocument/nsIContent, as well as parts of content/base/{public,src})
dom/window (nsGlobalWindow stuff currently in dom/src/base)
dom/events (including stuff currently in dom/src/events and content/events)
dom/geolocation
dom/json
dom/jsurl
dom/offline
dom/storage
dom/threads
dom/html
dom/html/canvas
dom/html/media
dom/mathml
dom/svg
dom/svg/smil
dom/xbl
dom/xml
dom/xslt
dom/xtf
dom/xul

Within these directories, avoid public/ and src/. In some cases I might keep
some subdirectories such as dom/xul/tree or dom/xul/templates to keep the
size of the directory manageable.

4) as a second step after the moves, stop exporting lots of internal headers
to dist/include. I think we probably need to keep exporting
nsIDocument/nsIContent, but avoid all the implementation headers.

I'm going to be doing this in small-ish pieces, for ease of testing/review.
`hg revert` doesn't un-do file moves correctly, so backing these changes out
after they have landed will be difficult and I'd like to avoid it if possible.

--BDS