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.

misc/schema/xrc_schema.rnc

@@ -152,6 +152,7 @@ builtinToplevelClasses =
 
 builtinWindowClasses =
     ( unknown
+    | wxActivityIndicator
     | wxAnimationCtrl
     | wxAuiNotebook
     | wxAuiToolBar
@@ -242,6 +243,7 @@ builtinClassesNames =
 
     | "unknown"
 
+    | "wxActivityIndicator"
     | "wxAnimationCtrl"
     | "wxAuiNotebook"
     | "wxAuiToolBar"
@@ -506,6 +508,15 @@ wxBitmap_or_wxIcon =
     }
 
 
+wxActivityIndicator =
+    element object {
+        attribute class { "wxActivityIndicator" } &
+        stdObjectNodeAttributes &
+        stdWindowProperties &
+        [xrc:p="o"] element running {_, t_bool }*
+    }
+
+
 wxAnimationCtrl =
     element object {
         attribute class { "wxAnimationCtrl" } &