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.

src/xrc/xh_activityindicator.cpp

@@ -0,0 +1,52 @@
+/////////////////////////////////////////////////////////////////////////////
+// Name:        src/xrc/xh_activityindicator.cpp
+// Purpose:     Implementation of wxActivityIndicator XRC handler.
+// Author:      Vadim Zeitlin
+// Created:     2015-03-18
+// Copyright:   (c) 2015 Vadim Zeitlin <vadim@wxwidgets.org>
+// Licence:     wxWindows licence
+/////////////////////////////////////////////////////////////////////////////
+
+// For compilers that support precompilation, includes "wx.h".
+#include "wx/wxprec.h"
+
+#ifdef __BORLANDC__
+    #pragma hdrstop
+#endif
+
+#if wxUSE_XRC && wxUSE_ACTIVITYINDICATOR
+
+#include "wx/xrc/xh_activityindicator.h"
+#include "wx/activityindicator.h"
+
+IMPLEMENT_DYNAMIC_CLASS(wxActivityIndicatorXmlHandler, wxXmlResourceHandler)
+
+wxActivityIndicatorXmlHandler::wxActivityIndicatorXmlHandler()
+{
+    AddWindowStyles();
+}
+
+wxObject *wxActivityIndicatorXmlHandler::DoCreateResource()
+{
+    XRC_MAKE_INSTANCE(ctrl, wxActivityIndicator)
+
+    ctrl->Create(m_parentAsWindow,
+                 GetID(),
+                 GetPosition(), GetSize(),
+                 GetStyle(),
+                 GetName());
+
+    SetupWindow(ctrl);
+
+    if ( GetBool(wxS("running")) )
+        ctrl->Start();
+
+    return ctrl;
+}
+
+bool wxActivityIndicatorXmlHandler::CanHandle(wxXmlNode *node)
+{
+    return IsOfClass(node, wxS("wxActivityIndicator"));
+}
+
+#endif // wxUSE_XRC && wxUSE_ACTIVITYINDICATOR