Support Shapes layer, for bounding box input

[cmalinmayor]

Open Design Questions:

• Do we want to support shapes other than rectangles? Circles? N-gons? What are the potential complications
• Do we want to support Shapes layers with different types of shapes mixed?
• If so, should different types of shapes be linked over time? Or should this be a user option?

This would involve:

• Determine answers to design questions above (what shapes to support and link)
• Create a new candidate_graph_from_shapes function (including computing IOU from shapes)
• Update the input layers list to include Shapes layers
• Implement a shapes input checker to ensure conformity to our requirements (e.g. no shapes spanning multiple timepoints)
• Call the correct candidate graph construction based on the input type of the layer
• Potentially implement a bounding box loader, because I am not sure how easy it is to get a list of bounding boxes into a napari Shapes layer