Add a method to let users determine if the current platform is X11.

In order to know if the methods in QtX11Extras are usable the user should
have a way to tell if they're running on X11.

Change-Id: I096267eb1f1fe07ee11811574a96100389c33b7d
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>