Keyboard shortcuts and other ergonomic considerations

[jni]

In moving from a specialised application to a napari plugin, we've lost some of @AbigailMcGovern's nice keyboard shortcuts for complex actions. I'm copying here the docstring of the corresponding class so we remember to implement something similar.

Correct labels to create a ground truth with five operations,
each of which correspond to the following number key.
(1) toggle selection of points - to seed watershed to split
labels.
(2) watershed - recompute watershed for joined labels based on
chosen points.
(3) toggle pick label colour - choose the colour of the label
with which to paint.
(4) toggle label fill mode - for merging labels
(5) toggle paint mode - paint new label.

Output can be saved to a new file by using Control-s. Note that this is
not necessary if using tensorstore, as annotations will be written to
file. However, this is still useful for generating separate copies of
GT data (if completing annotations of whole frames). If time_index is
None, the save operation will make a copy of the currently displayed
frame (dim = t) and the file will have a suffix of the form
"t_GT.zarr".