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/os2/notebook.h

@@ -69,11 +69,6 @@ public:
     // changes selected page without sending events
     int ChangeSelection(size_t nPage);
 
-    //
-    // Get the currently selected page
-    //
-    inline int      GetSelection(void) const { return m_nSelection; }
-
     //
     // Set/Get the title of a page
     //
@@ -193,8 +188,6 @@ protected:
     // Helper functions
     //
 
-    int                     m_nSelection; // The current selection (-1 if none)
-
 private:
     wxArrayLong                     m_alPageId;
     int                             m_nTabSize; // holds the largest tab size