[jQuery] possible jQuery ui icon bug

Run the "jQuery UI Sortable - Portlets Demo" and try swapping "ui-
icon-minusthick" for "ui-icon-plusthick", and you will notice that it
only seems to see the "ui-icon-minusthick" part of the image. This
appears to be the case for any pairing of icons where the second icon
appears to the right of the first icon in the ui-icons_*.png file.