Use global map for storing wxCocoaGesturesImpl

This decreases the speed of access to this struct (which shouldn't
matter that much) but avoids allocating an extra and almost always
unneeded pointer for each and every window, which seems like a good
trade-off.

include/wx/osx/cocoa/private.h

@@ -200,11 +200,6 @@ protected:
     // events, don't resend them
     bool m_hasEditor;
 
-#if MAC_OS_X_VERSION_MAX_ALLOWED >= MAC_OS_X_VERSION_10_10
-    // Data used for gesture support, if available.
-    class wxCocoaGesturesImpl* m_gesturesImpl;
-#endif // MAC_OS_X_VERSION_MAX_ALLOWED >= MAC_OS_X_VERSION_10_10
-
     wxDECLARE_DYNAMIC_CLASS_NO_COPY(wxWidgetCocoaImpl);
 };