Inspector plot: implement saving masks from drawn polygons

On the inspector plot, users would like to save masks from drawn rectangles or polygons somewhere, so they can be re-used for later reduction or other.

I imagine one way could be to save them as a DataGroup containing DataArrays with 0 and 1 values for the data (don't think you can make a DataArray with only masks and no data), or a DataArray with dummy values and a dict of masks. It would be good to keep the coordinates of the original data, so we can ensure masks are later applied on a compatible data array.

I'm currently thinking a function could be called on the inspector plot, and a small text box under the plot (text file to enter file name, and a Save button) could be added and would call that function.
So masks could be saved via widgets or programmatically.

Start by locating the inspector plot implementation and the existing rectangle or polygon drawing flow; no specific files or tests are named in the issue. Clarify the storage format and programmatic API before implementation. Done should include saving drawn masks, preserving the original data coordinates, and supporting later reuse through the proposed widget or API.