BIG WIP: feat: ROI model/view

[gselzer]

This PR provides a ROIModel class to maintain state about regions of interest, as well as a PRoiView protocol for views to implement when they want to react to changes in a ROIModel. This PR is by no means ready for merge, but exists for visibility and discussion about what is needed here.

For now, I'll edit mvc.py to include a (rectangular) ROI - next steps are to make it movable/resizable

Running points of discussion:

• What attributes are needed for the ROIModel? For a ND rectangular ROI, all we really need is minimum and maximum points in N dimensions, but thinking more broadly about lines, ellipses, freehand ROI selections, we might want more here. One idea I kind of like is an attribute that can provide a field of positions - something like imglib2's IterableRegion.
• Should ROIModel itself be a protocol that e.g. RectangularROIModel, LineROIModel, etc. implement?

[gselzer]

@tlambert03 one fun bug that just popped up for me is some strange rendering behavior if you run mvc.py, but then use the roi button to create a new ROI - parts of the rectangle border do not render (and the specific ones that do changed based on the window zoom), and the handles are occluded by the rectangle...kinda?

I'd be interested to see if you can reproduce 😅

[tlambert03]

I'd be interested to see if you can reproduce 😅

nope... wasn't able to see that