Improved hifi:// style URL scheme handling

First steps towards implementing #1463.

 - Moved url parsing code from Application::event to
   Menu::goTo(QString).
 - Handle destination component of URL as either a @username,
   #place, or x-xx,y-yy,z-zz style coordinate.
 - Added support for entering hifi:// style URL into @ invoked
   goto menu option.