Remove 11 heavy includes from Document.h that were only needed for
pointer/reference types (already forward-declared in Forward.h), and
extract the nested ViewportClient interface to a standalone header.
This reduces Document.h's recompilation cascade from ~1228 files to
~717 files (42% reduction). Headers like BrowsingContext.h that were
previously transitively included see even larger improvements (from
~1228 down to ~73 dependents).
This solves the issue where having a gamepad connected to a machine
running tests would cause different results for a few gamepad-related
tests.
Conveniently, all our fake testing gamepads are "virtual" in SDL, and
those are the only virtual ones.
We have to prevent from including any SDL headers in LibWeb headers.
Otherwise there will be transitive Windows.h includes that will
re-declare some of our existing forward decls/defines in
LibCore/SocketAddressWindows.h
