Add `chdir (base_dir)` to `__doc__`

Otherwise doctest fails if executed in any other directory than
`base_dir`.
Factor redundant CSS rules to Media_Defaults.Rule
Catch `ValueError` in `_handle_method_context`

To return `400 (bad request)` instead of `500 (internal server error)`
Add column `time` to table `change_history`

Add `DBW.db_sig` to `MOM.App_Type.db_sig` in order to detect changes
in the tables `change_history`, `pids`, and `scope_metadata`.
Add nested menus for attributes
Adapt jquery-ui dialog usage to 1.10 behavior