Fix #1124: A User is unable to reduce the size of a Resize node after plugging the resize with Y.Plugin.ResizeProxy

Hi @jonmak08,

This fixes the issue described in https://github.com/yui/yui3/issues/1124. Before shimming is added to ddm, the mousemove events are listened by `doc` and `shim`. When mouse is on top of the resize proxy, the mouse event can still be caught by `doc` when shim is covered by the resize proxy. After https://github.com/yui/yui3/commit/36a51b7f3a93ddb88272d3fd2ef83cff122cc846 is pushed in, mousemove can only be caught by `shim` when `useShim` is enabled. And since resize proxy has higher z-index than shim, the mousemove can't be caught when we drag the resize handle over proxy.So to fix this, I just give the shim a higher z-index. We can also change the z-index of the resize proxy, but like you said it's probably set for something. It's safer to change the z-index of the shim since the purpose of the shim is to track the mousemove events on the screen.

Let me know if there are any issues. Thanks!