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/core/private.h

@@ -326,6 +326,8 @@ public :
 
     virtual void        InstallEventHandler( WXWidget control = NULL ) = 0;
 
+    virtual bool        EnableTouchEvents(int eventsMask) = 0;
+
     // Mechanism used to keep track of whether a change should send an event
     // Do SendEvents(false) when starting actions that would trigger programmatic events
     // and SendEvents(true) at the end of the block.