Rename wxWebRequestEvent::GetResponseFileName() to GetDataFile()

This is shorter and doesn't imply that just the name (and not the full
path) is being returned.

Also rename wxWebResponse::GetFileName() to GetDataFile() for the same
reasons and for consistency. And document this previously undocumented
method.

include/wx/private/webrequest.h

@@ -151,7 +151,7 @@ public:
 
     wxString AsString() const;
 
-    virtual wxString GetFileName() const;
+    virtual wxString GetDataFile() const;
 
 protected:
     wxWebRequestImpl& m_request;