Toolbar vendor buttons

This allows anyone to add buttons to the toolbar, great for things like image upload.

It's done via the negation CSS3 pseudo-class `:not(X)`, which is not supported in IE 8 and lower ([overview](https://developer.mozilla.org/en-US/docs/Web/CSS/:not#Browser_compatibility)).

If IE 8 and lower support is important, I'll gladly fix it.