Openwith to handle function instead of URL

[will-moore]

See https://github.com/ome/omero-web/pull/232 for similar fixes in webclient.

If an open-with handler returns a function:

OME.setOpenWithUrlProvider("omero_figure", function (selected, url) {
     return function() {
        alert("Handle open...")
     }
});

This needs to happen ONLY when user clicks on the Open-with option, NOT when we simply render a link.