Add wxWindow::EnableTouchEvents()

Don't request touch event generation for all windows by default, this
has an inherent overhead and is not needed for 99% of the application
windows, so require calling EnableTouchEvents() explicitly to do it
instead.

Note that this requires properly initializing gesture recognizers in
wxOSX now that they're not always allocated, otherwise releasing them
when destroying the window would crash.

include/wx/osx/iphone/private.h

@@ -105,6 +105,7 @@ public :
     void                SetFont( const wxFont & font , const wxColour& foreground , long windowStyle, bool ignoreBlack = true );
 
     void                InstallEventHandler( WXWidget control = NULL );
+    bool                EnableTouchEvents(int WXUNUSED(eventsMask)) { return false; }
 
     virtual void        DoNotifyFocusEvent(bool receivedFocus, wxWidgetImpl* otherWindow);