Reuse wxBookCtrlBase::DoSetSelection() in wxTreebook too

Avoid duplicating base class DoSetSelection() implementation in
wxTreebook, just extend it slightly by using DoGetNonNullPage() to allow
using a (sub-)page if the page associated to the selected item is null
and reuse it.

Also get rid of wxTreebook::m_actualSelection, it seems completely
unnecessary to bother keeping and updating it when we can just find it
whenever we need (which actually seems to only have been the case in the
now removed DoSetSelection() implementation anyhow).

As a side effect of this, wxTreebook pages should now be sizer correctly
when switching to them as DoSetSelection() in the base class does call
SetSize() on the page before showing it, unlike the previously used
version in wxTreebook, which omitted this call for some reason.

There should be no other user-visible changes.

Closes #4379.

include/wx/treebook.h

@@ -38,7 +38,6 @@ public:
     // Default ctor doesn't create the control, use Create() afterwards
     wxTreebook()
     {
-        Init();
     }
 
     // This ctor creates the tree book control
@@ -49,8 +48,6 @@ public:
                long style = wxBK_DEFAULT,
                const wxString& name = wxEmptyString)
     {
-        Init();
-
         (void)Create(parent, id, pos, size, style, name);
     }
 
@@ -145,22 +142,16 @@ protected:
 
     // This subclass of wxBookCtrlBase accepts NULL page pointers (empty pages)
     virtual bool AllowNullPage() const wxOVERRIDE { return true; }
+    virtual wxWindow *DoGetNonNullPage(size_t page) wxOVERRIDE;
 
     // event handlers
     void OnTreeSelectionChange(wxTreeEvent& event);
     void OnTreeNodeExpandedCollapsed(wxTreeEvent& event);
 
-    // array of page ids and page windows
+    // array of tree item ids corresponding to the page indices
     wxVector<wxTreeItemId> m_treeIds;
 
-    // in the situation when m_selection page is not wxNOT_FOUND but page is
-    // NULL this is the first (sub)child that has a non-NULL page
-    int m_actualSelection;
-
 private:
-    // common part of all constructors
-    void Init();
-
     // The real implementations of page insertion functions
     // ------------------------------------------------------
     // All DoInsert/Add(Sub)Page functions add the page into :
@@ -182,12 +173,11 @@ private:
                          bool bSelect = false,
                          int imageId = NO_IMAGE);
 
-    // Sets selection in the tree control and updates the page being shown.
-    int DoSetSelection(size_t pos, int flags = 0) wxOVERRIDE;
-
-    // Returns currently shown page. In a case when selected the node
-    // has empty (NULL) page finds first (sub)child with not-empty page.
-    wxTreebookPage *DoGetCurrentPage() const;
+    // Overridden methods used by the base class DoSetSelection()
+    // implementation.
+    void UpdateSelectedPage(size_t newsel) wxOVERRIDE;
+    wxBookCtrlEvent* CreatePageChangingEvent() const wxOVERRIDE;
+    void MakeChangedEvent(wxBookCtrlEvent &event) wxOVERRIDE;
 
     // Does the selection update. Called from page insertion functions
     // to update selection if the selected page was pushed by the newly inserted