Napari Shortcuts for updating labels

[ianhi]

• Swap labels of two cells
• scroll timepoints with left hand
  • Overhaul reindexing + add tests + misc improvements #11 adds Shift+scroll
• drilldown for labelling/erasing
• temporary hold down key for fill with zero
  • Overhaul reindexing + add tests + misc improvements #11 Adds this as s
• eyedropper that swaps to paint mode as soon as clicked

[ianhi]

https://forum.image.sc/t/labeling-with-paintbrush-over-n-dims/48581