Reuse base class helper in wxGTK wxNotebook::InsertPage()

Call DoSetSelectionAfterInsertion() instead of partially duplicating it.

This ensures that m_selection is correct after inserting a new page,
which wasn't the case before. Surprisingly, this didn't actually matter
because wxGTK implementation doesn't really use the base class
m_selection neither and just queries GTK+ widget directly for its
selection in GetSelection() instead, but it was at the very least
confusing.

src/gtk/notebook.cpp

@@ -483,10 +483,7 @@ bool wxNotebook::InsertPage( size_t position,
     }
 #endif
 
-    if (select && GetPageCount() > 1)
-    {
-        SetSelection( position );
-    }
+    DoSetSelectionAfterInsertion(position, select);
 
     InvalidateBestSize();
     return true;