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/generic/notebook.h

@@ -65,8 +65,6 @@ public:
   int SetSelection(size_t nPage);
     // cycle thru the tabs
   //  void AdvanceSelection(bool bForward = true);
-    // get the currently selected page
-  int GetSelection() const { return m_nSelection; }
 
     // changes selected page without sending events
   int ChangeSelection(size_t nPage);
@@ -149,8 +147,6 @@ protected:
   // helper functions
   void ChangePage(int nOldSel, int nSel); // change pages
 
-  int m_nSelection;           // the current selection (-1 if none)
-
   wxTabView*   m_tabView;
 
   DECLARE_DYNAMIC_CLASS(wxNotebook)