Add wxDataViewCtrl::ExpandChildren()

This convenient method allows to expand all children of the item
recursively.

This is directly supported by both native implementations, so it makes
sense to have this in wxDataViewCtrl itself, even if it could be also
(less efficiently) implemented in the user code.

interface/wx/dataview.h

@@ -1402,6 +1402,16 @@ public:
     */
     void ExpandAncestors( const wxDataViewItem & item );
 
+    /**
+        Expand all all children of the given item recursively.
+
+        This is the same as calling Expand() on the @a item itself and then
+        calling it for all of its children, grandchildren etc recursively.
+
+        @since 3.1.5
+     */
+    void ExpandChildren( const wxDataViewItem & item );
+
     /**
         Returns pointer to the column. @a pos refers to the position in the
         control which may change after reordering columns by the user.