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/treebook.h

@@ -126,7 +126,6 @@ public:
     // Standard operations inherited from wxBookCtrlBase
     // -------------------------------------------------
 
-    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;
@@ -152,9 +151,6 @@ protected:
     // array of page ids and page windows
     wxArrayTreeItemIds m_treeIds;
 
-    // the currently selected page or wxNOT_FOUND if none
-    int m_selection;
-
     // in the situation when m_selection page is not wxNOT_FOUND but page is
     // NULL this is the first (sub)child that has a non-NULL page
     int m_actualSelection;