This allows more consistency in the manner we include core headers,
where previously there would be a mix of absolute, relative and
include path-dependent includes.
Using `misc/scripts/fix_headers.py` on all Godot files.
Some missing header guards were added, and the header inclusion order
was fixed in the Bullet module.
This makes the interfaces available, without implementation, in other
platforms and the editor, which facilitates documenting platform-exclusive
classes.
Platform-exclusive APIs must be set up in platform/<platform>/api/api.cpp.
Provide noop method-implementations where necessary.
Also setup and document the HTML5 platform's JavaScript singleton.
