CLOUD: Add complex callbacks

Originally, I intended to add Storage API, StorageFile and StorageInfo
stubs. When I tried to implement a simple info() call, I ended up fixing
Request to contain some pointer field and all callbacks to have Request*
parameter. And, now I have to place callback pointer into Request. which
calls another callback.

And, eventually, these "simple" callbacks would again require another
pointer (to some caller class).

backends/networking/curl/curljsonrequest.cpp

@@ -75,7 +75,7 @@ bool CurlJsonRequest::handle() {
 			if (_callback) {
 				char *contents = getPreparedContents();				
 				Common::JSONValue *json = Common::JSON::parse(contents);				
-				_callback(json); //potential memory leak, free it in your callbacks!
+				_callback(this, json); //potential memory leak, free it in your callbacks!
 			}
 			return true;
 		}