Fix for Bug 5593, Change cart markup to allow for easier icon styling

Fix for Bug 5593, Change cart markup to allow for easier icon styling

Replacing inline cart image with a span with a unique ID. Adding
styling to the deafult CSS to give that span a background of the
same cart image as before. Custom CSS can override this with a
different image.

Example for OPACUserCSS preference:

  background-image : url("/opac-tmpl/prog/famfamfam/silk/basket.png");
  width:16px;
  height:16px;
}

Signed-off-by: Katrin Fischer <Katrin.Fischer.83@web.de>
    background-image: url("/opac-tmpl/prog/famfamfam/silk/basket.png");
}

Will also change the cart image in opac detail view.
Signed-off-by: Chris Cormack <chrisc@catalyst.net.nz>