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/toolbook.h

@@ -70,7 +70,6 @@ public:
 
 
     // implement base class virtuals
-    virtual int GetSelection() const;
     virtual bool SetPageText(size_t n, const wxString& strText);
     virtual wxString GetPageText(size_t n) const;
     virtual int GetPageImage(size_t n) const;
@@ -110,9 +109,6 @@ protected:
     wxBookCtrlEvent* CreatePageChangingEvent() const;
     void MakeChangedEvent(wxBookCtrlEvent &event);
 
-    // the currently selected page or wxNOT_FOUND if none
-    int m_selection;
-
     // whether the toolbar needs to be realized
     bool m_needsRealizing;