Fix dd in ie6, 7

Fix for https://github.com/yui/yui3/issues/1971

This pull request rolls back https://github.com/yui/yui3/pull/1831 which breaks dd in ie 6 and 7.

As I understand it, the https://github.com/yui/yui3/pull/1831 fix addresses an edge case in panel while breaking core functionality in dd.

In my testing, the edge case was only fixed in chrome and safari, I could still get past modal in firefox.
Additionally, the steps taken to reproduce the panel issue completely break panel in ie 6 - 10. This is not addressed.

@ispyinternet
@andrewnicols
Please chime in if I'm missing something. Thanks.