Widgets

This PR adds visualization widgets for the supported deformable models. More specifically, it adds:
* `visualize_shape_model`
* `visualize_appearance_model`
* `visualize_aam`
* `visualize_atm`
* `visualize_fitting_results`
* `plot_ced`

Finally, the `AAM`, `ATM`, `CLM` and `FittingResult` class have a `view_widget()` method that calls the corresponding widget (`visualize_aam`, `visualize_atm`, `visualize_shape_model` and `visualize_fitting_results` respectively).