Override DoEnable() instead of Enable() in wxGTK controls

This is slightly simpler, as it doesn't require checking whether the
control state really changes or not (it always does if DoEnable() is
called) and allows disabling the controls before creating them, e.g.
code like

    wxButton* const b = new wxButton();
    b->Disable();
    b->Create(this, wxID_OK);

works as expected now instead of spewing GTK+ errors.

src/gtk/textctrl.cpp

@@ -1350,17 +1350,14 @@ void wxTextCtrl::SetEditable( bool editable )
     }
 }
 
-bool wxTextCtrl::Enable( bool enable )
+void wxTextCtrl::DoEnable(bool enable)
 {
-    if (!wxWindowBase::Enable(enable))
-    {
-        // nothing to do
-        return false;
-    }
+    if ( !m_text )
+        return;
+
+    wxTextCtrlBase::DoEnable(enable);
 
     gtk_widget_set_sensitive( m_text, enable );
-
-    return true;
 }
 
 void wxTextCtrl::MarkDirty()