Don't include wx/treectrl.h from wx/treebook.h any more

Remove an unnecessary header dependency. This is not completely
backwards-compatible as it would break any code relying on getting e.g.
wxTextCtrl declaration after including wx/treebook.h, but, hopefully,
there shouldn't be that much such code out there and fixing it shouldn't
be difficult.

If either of these assumptions turns out to be false, this commit can
always be reverted later.

docs/changes.txt

@@ -63,6 +63,9 @@ Changes in behaviour which may result in build errors
   This only affects code defining its own custom renderers, code just using
   wxGraphicsContext::CreatePen() continues to compile and work as before.
 
+- wx/treebook.h doesn't include wx/treectrl.h (and, via it, wx/textctrl.h) any
+  more, include these headers explicitly from your code if necessary.
+
 
 3.1.2: (released 2018-??-??)
 ----------------------------

include/wx/treebook.h

@@ -17,10 +17,11 @@
 
 #include "wx/bookctrl.h"
 #include "wx/containr.h"
-#include "wx/treectrl.h"        // for wxArrayTreeItemIds
+#include "wx/treebase.h"        // for wxTreeItemId
 
 typedef wxWindow wxTreebookPage;
 
+class WXDLLIMPEXP_FWD_CORE wxTreeCtrl;
 class WXDLLIMPEXP_FWD_CORE wxTreeEvent;
 
 // ----------------------------------------------------------------------------