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.

interface/wx/event.h

@@ -3649,6 +3649,10 @@ public:
 /** @class wxGestureEvent
     This is the base class for all supported gesture events.
 
+    @note Gesture events are not generated by default, you must call
+          wxWindow::EnableTouchEvents() with the appropriate parameter to
+          request their generation.
+
     @library{wxcore}
     @category{events}