Move workaround for initial TLW size to wxGTK itself

Add wxWindow::WXSetInitialFittingClientSize() instead of handling wxGTK
TLWs specially in the common wxSizer code and override it in wxGTK to
remember that we need to reset the client size once the window is shown.

This commit shouldn't result in any changes in the observed behaviour.

include/wx/gtk/toplevel.h

@@ -138,6 +138,16 @@ public:
 
     void GTKDoAfterShow();
 
+#ifdef __WXGTK3__
+    void GTKUpdateClientSizeIfNecessary();
+
+    virtual void WXSetInitialFittingClientSize(int flags) wxOVERRIDE;
+
+private:
+    // Flags to call WXSetInitialFittingClientSize() with if != 0.
+    int m_pendingFittingClientSizeFlags;
+#endif // __WXGTK3__
+
 protected:
     // give hints to the Window Manager for how the size
     // of the TLW can be changed by dragging