implement Layout() so that sizers work correctly when the window is scrolled (1st part of the patch 1439905)

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@38780 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/vscroll.h

@@ -135,6 +135,8 @@ public:
     // is kept for backwards compatibility
     size_t GetLastVisibleLine() const { return GetVisibleEnd() - 1; }
 
+    // layout the children (including the sizer if needed)
+    virtual bool Layout();
 
 protected:
     // this function must be overridden in the derived class and it should