Add wxWindow::CallForEachChild()

Adding such function was discussed a few times in the past and it's
going to be needed by wxWidgets itself in the next commit, so do add it,
finally, so that it could also be used from the application code too.

include/wx/window.h

@@ -784,6 +784,10 @@ public:
     // needed just for extended runtime
     const wxWindowList& GetWindowChildren() const { return GetChildren() ; }
 
+        // call the specified functor for all children, recursively
+    template <typename T>
+    void CallForEachChild(const T& fn);
+
         // get the window before/after this one in the parents children list,
         // returns nullptr if this is the first/last window
     wxWindow *GetPrevSibling() const { return DoGetSibling(OrderBefore); }
@@ -2057,6 +2061,15 @@ inline wxWindow *wxWindowBase::GetGrandParent() const
     return m_parent ? m_parent->GetParent() : nullptr;
 }
 
+template <typename T>
+void wxWindowBase::CallForEachChild(const T& fn)
+{
+    fn(static_cast<wxWindow*>(this));
+
+    for ( auto& child : GetChildren() )
+        child->CallForEachChild(fn);
+}
+
 // ----------------------------------------------------------------------------
 // global functions
 // ----------------------------------------------------------------------------

interface/wx/window.h

@@ -592,6 +592,30 @@ public:
     */
     virtual void AddChild(wxWindow* child);
 
+    /**
+        Invoke the given functor for all children of the given window
+        recursively.
+
+        This function calls @a functor for the window itself and then for all
+        of its children, recursively.
+
+        Example of using it for implementing a not very smart translation
+        function:
+        @code
+        void MyFrame::OnTranslate(wxCommandEvent&) {
+            CallForEachChild([](wxWindow* win) {
+                wxString rest;
+                if ( win->GetLabel().StartsWith("Hello ", &rest) )
+                    win->SetLabel("Bonjour " + rest);
+            });
+        }
+        @endcode
+
+        @since 3.3.0
+     */
+    template <typename T>
+    void CallForEachChild(const T& functor);
+
     /**
         Destroys all children of a window. Called automatically by the destructor.
     */