Add a hook to suppress themekey evaluation on special paths

Hi,
I noticed that themekey is evaluated on requests to
js/admin_menu/cache/...

This is probably no biggie in itself. But if there is an expensive rule at the top, we will get a performance penalty.
The effect is similar for other non-html requests or for ajax.

For ajax, I think it is more difficult: We probably want to get the same theme as on the embedding page.
Even on other non-html or ajax pages, the theme choice might still be relevant.

So.. how can we solve this?

I would suggest
- so something smart for ajax requests, to use the same theme as on the embedding page.
- by default, put a themekey rule at the top targeting js/admin_menu, to suppress any further rules evaluation.

Ok, this is probably not the final wisdom. Maybe you have better ideas (or something is already implemented).