Create RFB object on connect

Create RFB object on connect

In e543525faa9cf0d683f41e183e89cd909f3dd229, we switched to creating
a new RFB object on disconnect.  This caused issues, however, since
any errors were only displayed briefly before the new "loaded" text
was displayed instead.

Now, we create the RFB object on connect.  This essentially removes
the usefulness of the "loaded" state, but prevents the aforementioned
problem.

To facilitate this, the code which does detection of cursor URI support
was moved from this Display constructor (which now calls the new
function) into its own function, `Util.browserSupportsCursorURIs()`.

Fixes #467