Update wxBookCtrlBase::m_selection outside UpdateSelectedPage()

Change m_selection in wxBookCtrlBase::DoSetSelection() itself instead of
requiring all the derived class overriding do it in their overridden
UpdateSelectedPage().

No real changes, this is just a small simplification.

include/wx/choicebk.h

@@ -86,8 +86,7 @@ protected:
 
     void UpdateSelectedPage(size_t newsel) wxOVERRIDE
     {
-        m_selection = static_cast<int>(newsel);
-        GetChoiceCtrl()->Select(m_selection);
+        GetChoiceCtrl()->Select(newsel);
     }
 
     wxBookCtrlEvent* CreatePageChangingEvent() const wxOVERRIDE;

include/wx/simplebook.h

@@ -155,9 +155,10 @@ public:
     }
 
 protected:
-    virtual void UpdateSelectedPage(size_t newsel) wxOVERRIDE
+    virtual void UpdateSelectedPage(size_t WXUNUSED(newsel)) wxOVERRIDE
     {
-        m_selection = (int)newsel;
+        // Nothing to do here, but must be overridden to avoid the assert in
+        // the base class version.
     }
 
     virtual wxBookCtrlEvent* CreatePageChangingEvent() const wxOVERRIDE

src/common/bookctrl.cpp

@@ -500,6 +500,7 @@ int wxBookCtrlBase::DoSetSelection(size_t n, int flags)
             DoShowPage(page, true);
 
             // change selection now to ignore the selection change event
+            m_selection = n;
             UpdateSelectedPage(n);
 
             if ( flags & SetSelection_SendEvent )

src/generic/listbkg.cpp

@@ -293,7 +293,6 @@ void wxListbook::SetImageList(wxImageList *imageList)
 
 void wxListbook::UpdateSelectedPage(size_t newsel)
 {
-    m_selection = newsel;
     GetListView()->Select(newsel);
     GetListView()->Focus(newsel);
 }

src/generic/toolbkg.cpp

@@ -197,7 +197,6 @@ void wxToolbook::MakeChangedEvent(wxBookCtrlEvent &event)
 
 void wxToolbook::UpdateSelectedPage(size_t newsel)
 {
-    m_selection = newsel;
     GetToolBar()->ToggleTool(newsel + 1, true);
 }