Add a tool to mark rectangular (any shape?) regions and label them. Then we should be able to export the specific slice/region using #645.

• add --list-slices (print slices on output or/and export to json file)
• support to export only the area of a specific slice (e.g. --slice name)
• support to export all slices like --save-as sheet-{slice}.png (like a --split-slices)
• animation support for slices
• review Quality of Life improvements for working with slices #1651