Add wxComboBox::MSWRecreate() and use it from wxBitmapComboBox

No real changes, this is just a refactoring moving the code recreating
the native MSW combobox into wxComboBox itself, as it seems better to do
this in this class itself rather than outside of it.

This commit is best viewed with Git --color-moved option.

include/wx/msw/combobox.h

@@ -154,6 +154,12 @@ protected:
         m_allowTextEvents = enable;
     }
 
+    // Recreate the native control entirely while preserving its initial
+    // contents and attributes: this is useful if the height of the items must
+    // be changed as the native control doesn't seem to support doing this once
+    // it had been already determined.
+    void MSWRecreate();
+
 private:
     // there are the overridden wxTextEntry methods which should only be called
     // when we do have an edit control so they assert if this is not the case