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/common/bookctrl.cpp

@@ -53,6 +53,7 @@ END_EVENT_TABLE()
 
 void wxBookCtrlBase::Init()
 {
+    m_selection = wxNOT_FOUND;
     m_bookctrl = NULL;
     m_imageList = NULL;
     m_ownsImageList = false;