Widgets fixes

This PR solves two bugs in widgets:

1. `visualize_images` now supports images without landmarks. The user can also pass a mixed list of landmarked, non-landmarked, masked and multichannel images.
2. `visualize_fitting_results` now visualizes results that do not have a groundtruth shape (`gt_shape` is `None`). The user can pass a list of mixed fitting result objects that either have or not a gt shape.