Custom URL handlers for the vgui::HTML panel now pass the URL and protocol.

In Source 2007, the vgui::HTML panel passed a KeyValues object containing `url` and `protocol`.

For custom URLs such as `test://foobar`, `url` would be `foobar` and `protocol` would be `test`.

In Source 2013, `protocol` was missing, and `url` was set to the protocol.

This was originally refused by Valve (https://github.com/ValveSoftware/source-sdk-2013/pull/116), so I thought it would find a happier home here.