Ensure wxWebRequest is in idle state before starting it

Check that current state is State_Idle in wxWebRequest itself only once
instead of doing it in 2 (out of 3) wxWebRequestImpl implementations.

Also assert if this is not the case instead of silently doing nothing
which would surely be more difficult to debug.

interface/wx/webrequest.h

@@ -187,6 +187,9 @@ public:
 
         Events will be triggered on success or failure.
 
+        The current state must be @c State_Idle, already started requests can't
+        be started again.
+
         @see Cancel()
     */
     void Start();