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_activityindicator.h

@@ -0,0 +1,31 @@
+/////////////////////////////////////////////////////////////////////////////
+// Name:        wx/xrc/xh_activityindicator.h
+// Purpose:     Declaration of wxActivityIndicator XRC handler.
+// Author:      Vadim Zeitlin
+// Created:     2015-03-18
+// Copyright:   (c) 2015 Vadim Zeitlin <vadim@wxwidgets.org>
+// Licence:     wxWindows licence
+/////////////////////////////////////////////////////////////////////////////
+
+#ifndef _WX_XH_ACTIVITYINDICATOR_H_
+#define _WX_XH_ACTIVITYINDICATOR_H_
+
+#include "wx/xrc/xmlres.h"
+
+#if wxUSE_XRC && wxUSE_ACTIVITYINDICATOR
+
+class WXDLLIMPEXP_XRC wxActivityIndicatorXmlHandler : public wxXmlResourceHandler
+{
+public:
+    wxActivityIndicatorXmlHandler();
+
+    virtual wxObject *DoCreateResource() wxOVERRIDE;
+    virtual bool CanHandle(wxXmlNode *node) wxOVERRIDE;
+
+private:
+    wxDECLARE_DYNAMIC_CLASS(wxActivityIndicatorXmlHandler);
+};
+
+#endif // wxUSE_XRC && wxUSE_ACTIVITYINDICATOR
+
+#endif // _WX_XH_ACTIVITYINDICATOR_H_