Add Spinbox with unbound values

[tlambert03]

Python int type is unbounded, so it would be nice to have a SpinBox that represents that.

Could be done (in qt at least) using QAbstractSpin box, leaving the model to python.

requested here: https://forum.image.sc/t/visualizing-feature-measurements-in-napari-using-colormaps-as-luts/51567/21?u=talley

[imagesc-bot]

This issue has been mentioned on Image.sc Forum. There might be relevant details there:

https://forum.image.sc/t/visualizing-feature-measurements-in-napari-using-colormaps-as-luts/51567/25