MDL-43853 Atto: Performance. Turbocharge the loader.

Rewrite the atto plugin loader so that:

A) We have a real settings.php page and can edit the toolbar.
B) All plugins are loaded at the same time (not serially)
C) All plugins share the YUI sandbox (more performance boost).

I had to change the loading API for this - but there are not non-core uses yet,
this is the best time to break it.