Add wxTreeCtrl::SelectChildren() method.

Add MSW and generic implementation, documentation and change to the sample
showing it.

Closes #11620.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@63277 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

interface/wx/treectrl.h

@@ -954,6 +954,15 @@ public:
         Unselects the given item. This works in multiselection controls only.
     */
     void UnselectItem(const wxTreeItemId& item);
+
+    /**
+        Select all the immediate children of the given parent.
+
+        This function can be used with multiselection controls only.
+
+        @since 2.9.1
+    */
+    virtual void SelectChildren(const wxTreeItemId& parent);
 };