The API is implemented in javascript, and generates C functions that can
be called from godot.
This allows much cleaner code replacing all `EM_ASM` calls in our C++
code with plain C function calls.
This also gets rid of few hacks and comes with few optimizations (e.g.
custom cursor shapes should be much faster now).
A new editor plugin, specific to HTML5, that provide some extra features
needed to make the editor usable on that platform.
For now, it adds a "Download project sources" option in the "Tool" menu,
so the user can download the work done as a zip file (from the browser
storage).
