Add interactive masking tool

[nvaytet]

Part of https://github.com/scipp/essdiffraction/issues/50

• Masks can be drawn on 1d and 2d data (using mpltoolbox).
• They can then be saved to file, using Scipp's hdf5 format.
• Only rectangular and horizontal/vertical spans are supported for now.

[nvaytet]

To be moved to scippneutron