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/univ/notebook.h

@@ -64,7 +64,6 @@ public:
     // --------------------------------------
 
     virtual int SetSelection(size_t nPage) { return DoSetSelection(nPage, SetSelection_SendEvent); }
-    virtual int GetSelection() const { return (int) m_sel; }
 
     // changes selected page without sending events
     int ChangeSelection(size_t nPage) { return DoSetSelection(nPage); }
@@ -210,9 +209,6 @@ protected:
     // the pages titles
     wxArrayString m_titles;
 
-    // the current selection
-    size_t m_sel;
-
     // the spin button to change the pages
     wxSpinButton *m_spinbtn;