Refactor: use wxBookCtrlBase::m_selection in all derived classes.

All book control classes with the exception of wxGTK wxNotebook stored the
currently selected page in m_selection or m_nSelection (or, in wxUniv
wxNotebook case, m_sel) variable. Remove all of them and add m_selection
directly to the base class itself so that it can be reused everywhere.

Closes #12622.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@65931 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/gtk/notebook.h

@@ -110,6 +110,9 @@ public:
     // common part of all ctors
     void Init();
 
+    // Called by GTK event handler when the current page is definitely changed.
+    void GTKOnPageChanged();
+
     // helper function
     wxGtkNotebookPage* GetNotebookPage(int page) const;