Implement conversion function between `napari` Shapes layer and our ROI classes

[niksirbi]

In #617, I've drafted a napari widget for creating regions of interest as shapes, and assigning names to them. This widget is currently missing Save/Load buttons, because we have no way to natively save/load movement ROIs to disk, see #675.

There is however a missing link, we also need functions for converting between napari Shapes (the format in which ROIs are drawn) and RegionOfInterest objects (the format in which ROIs are represented in movement.

This is necessary for linking the save/load buttons of the ROI drawing widget to the methods proposed in #675.

I plan to tackle this myself this fall, as part of the larger #378 effort.