Provide a widget for drawing regions-of-interest (ROIs) in napari

[niksirbi]

After we've figured out how to represent ROIs in movement—see issue #377, and we've released a minimal version of our napari plugin—see issue #31, we should add a napari widget for defining ROIs.

We'd probably end up working with the napari Shapes layer and the widget would handle the conversions between that format and whatever ROI representation we choose in movement.