1070585: GUI no longer locks on connection test. Adds cancel button.

Fixes: https://bugzilla.redhat.com/show_bug.cgi?id=1070585

Seems odd to set self.proxy_user/proxy_password/proxy_prefix here. Not sure the side effects are intentional.

maybe like?
```
    unused, unused, self.proxy_host, self.proxy_port, unused = parse_url(proxy_url, default_port=rhsm.confi...
```

or 
```
  url_parts= parse_url(proxy_url, default_port=rhsm.confi...
  # Just need the host and port here, 
  self.proxy_host = url_parts[2]
  self.proxy_port = url_parts[3]
```

In hingsight, parse_url() should have returned a NamedTuple, but alas.