Make wxTopLevelWindow::Layout() do the expected thing

It makes sense for explicit calls to Layout() to use the same logic as
is implicitly used when a TLW is resized, so override it to use
TLW-specific logic instead of using a separate DoLayout() for this.

Note that DoLayout() still has to be preserved because Debian code
search finds at least a couple of examples of its use outside the
library, meaning that there are probably quite a few more of them in the
wild.

Also, wxTopLevelWindow still needs its own wxEVT_SIZE handler because
the base class only calls Layout() if GetAutoLayout() is true, while we
want it to be always called. This might be worked around by just calling
SetAutoLayout(true) in wxTopLevelWindow ctor, but it seems safer, from
compatibility point of view, to keep wxTopLevelWindow::OnSize() instead.

See #18472.

include/wx/toplevel.h

@@ -276,9 +276,13 @@ public:
     virtual bool IsTopNavigationDomain(NavigationKind kind) const wxOVERRIDE;
     virtual bool IsVisible() const { return IsShown(); }
 
+    // override to do TLW-specific layout: we resize our unique child to fill
+    // the entire client area
+    virtual bool Layout() wxOVERRIDE;
+
     // event handlers
     void OnCloseWindow(wxCloseEvent& event);
-    void OnSize(wxSizeEvent& WXUNUSED(event)) { DoLayout(); }
+    void OnSize(wxSizeEvent& WXUNUSED(event)) { Layout(); }
 
     // Get rect to be used to center top-level children
     virtual void GetRectForTopLevelChildren(int *x, int *y, int *w, int *h);
@@ -326,9 +330,8 @@ protected:
     // send the iconize event, return true if processed
     bool SendIconizeEvent(bool iconized = true);
 
-    // do TLW-specific layout: we resize our unique child to fill the entire
-    // client area
-    void DoLayout();
+    // this method is only kept for compatibility, call Layout() instead.
+    void DoLayout() { Layout(); }
 
     static int WidthDefault(int w) { return w == wxDefaultCoord ? GetDefaultSize().x : w; }
     static int HeightDefault(int h) { return h == wxDefaultCoord ? GetDefaultSize().y : h; }