Inspector plot: implement saving masks from drawn polygons

[nvaytet]

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.