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.

docs/doxygen/overviews/xrc_format.h

@@ -541,6 +541,14 @@ control-specific properties are listed. If the control can have child objects,
 it is documented there too; unless said otherwise, XRC elements for these
 controls cannot have children.
 
+@subsubsection xrc_activityindicator wxActivityIndicator
+
+@beginTable
+@hdr3col{property, type, description}
+@row3col{running, @ref overview_xrcformat_type_bool,
+    If true, start the activity indicator after creating it (default: false).}
+@endTable
+
 @subsubsection xrc_wxanimationctrl wxAnimationCtrl
 
 @beginTable