Add XRC handler for wxActivityIndicator control.

The only attribute for this control is a boolean "running" which can be set to
start the indicator on load.

Update the schema, documentation and the XRC sample.

include/wx/xrc/xh_all.h

@@ -13,6 +13,7 @@
 
 // Existing handlers:
 
+#include "wx/xrc/xh_activityindicator.h"
 #include "wx/xrc/xh_animatctrl.h"
 #include "wx/xrc/xh_bannerwindow.h"
 #include "wx/xrc/xh_bmp.h"