Improve the performance and functionality of the whatever:hover behavior

Currently whatever:hover has terrible performance, because addRule causes IE to re-parse the whole skin and re-style the whole document, so the total time increases both with the number of :hover rules and the size of the HTML. This usually means 5-6 seconds of dead time in IE6 and 7.

As functionality, it does not support more than one class used on an element (.a:hover and .b:hover where both selectors match the same element), plus other rarely used scenarios.