output segmented result as Labels layer

## Requested feature
*Describe the new feature you would like to see in the napari-segmenter plugin.*
Currently the segmentation outputs are saved as images with True/False values. The alternative is to save segmented result as labels layer, which assigns 0 as background and integers for objects. Saving as labels layer would allow direct property value extraction (for example using https://www.napari-hub.org/plugins/napari-skimage-regionprops).

## Use case
*What will this change enable you to do?*
To allow quantitative analysis of segmented result more directly. Also to enable possibly object segmentation not just semantic segmentation.

## If this change is NOT made...
Check all that apply, by entering an x inside the brackets:
- [ ] I will not use the napari-segmenter plugin.
- [ ] I will be blocked from completing my current tasks.
- [x] I can accomplish what I need to accomplish, but it's tedious.
- [ ] Not a problem right this minute, but this is going to be a problem soon.
- [ ] Everything's fine! But it would be cool if this work happened at some point in the future.

## Alternatives
*Are there any other plugins or software that you would use in place of this feature?*
napari has Image Layer (right click) > Convert to Labels option to make the conversion, but could be tedious to do so if working with many images.