fix: clims sliders not aligned with bit depth

[gselzer]

If I put an unsigned 8-bit dataset into ndv, the range of the clims slider would ideally be [0, 255], but they are hardcoded to [0, 65535].

There's TODOs (e.g. here) about this. but not a concrete issue that I found.

I think this is best solved by a LutView.set_extremes() method (naming suggestions welcome) that defines the min and max - the histograms would benefit from this as well (as I added something similar to the histogram canvas in #146)

[gselzer]

Forgot to add an image in action: