Move wxGTK gesture-related data in a private opaque struct

This is a first step towards enabling gesture events only for the
windows that are interested in them as it will make it possible to avoid
wasting space on unused data in the windows that don't need it.

No real changes so far.

include/wx/gtk/window.h

@@ -356,14 +356,9 @@ public:
     wxRegion             m_nativeUpdateRegion;  // not transformed for RTL
 
 #if defined(__WXGTK3__)
-    unsigned int         m_touchCount;
-    unsigned int         m_lastTouchTime;
-    int                  m_gestureState;
-    int                  m_allowedGestures;
-    int                  m_activeGestures;
-    wxPoint              m_lastTouchPoint;
-    GdkEventSequence*    m_touchSequence;
-#endif
+    // Data used for gesture support, if available.
+    class wxWindowGesturesData* m_gesturesData;
+#endif // __WXGTK3__
 
 protected:
     // implement the base class pure virtuals