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

src/generic/treebkg.cpp

@@ -556,11 +556,6 @@ bool wxTreebook::SetPageImage(size_t n, int imageId)
     return true;
 }
 
-int wxTreebook::GetSelection() const
-{
-   return m_selection;
-}
-
 int wxTreebook::DoSetSelection(size_t pagePos, int flags)
 {
     wxCHECK_MSG( IS_VALID_PAGE(pagePos), wxNOT_FOUND,