Improve labels transfer to ImageJ

[gselzer]

This PR provides the ability to transfer napari Labels to the ImageJ UI. Notably, it provides two different pathways:

1. As the image: When a Labels layer is provided as the image, napari-imagej will convert the Labels layer to an ImgLabeling, and then will display the index image.
2. As rois: When a Labels layer is provided as the rois, napari-imagej will convert the Labels layer to ROITree (using @elevans' wonderful work in Add conversion support for index images to ImgLib2 and legacy ImageJ ROIs pyimagej#283), then attached and displayed as ROIs. Note that this pathway required the bump in the pyimagej min version.

You can see the results in the image below, where I:

1. Loaded in the brick sample image in napari
2. Manually annotated two labels on a new Layer
3. Using the detailed image transfer button, transferred the Labels as an image to get (V) in the bottom left corner
4. Using the detailed image transfer button, transferred brick as an image and Labels as rois to get brick (V) in the bottom right corner.

Concerns

Working around imagej-common: Arguably imagej-common (maybe here) and/or imagej-legacy should shoulder the burden of correctly displaying ImgLabelings. I decided to avoid an implementation there because (a) it would take longer to be usable from napari-imagej and (b) I had a feeling the changes could be involved. @hinerm thoughts on this?

Closes #321 (cc @ian-coccimiglio)

[ian-coccimiglio]

Now we're cooking - this is great.