Max Merge option in Frame Selector

[annehaley]

Addresses #1118

[manthey]

This looks like the right approach. In band compositing mode on the normmedia_8well_col2_livecellgfp.nd2 test file, I see nulls being send for the framedelta.

And, one further wrinkle: we really should be showing the histogram of the max-merged image, not the un-merged image. This means that we need to pass the appropriate style to the histogram endpoint when max-merge is checked.

[manthey]

This is working, though there is an eccentricity in non-8-bit images. Specifically, when a style gets applied, the histogram ends up in 8 bits, which then breaks the sliders setting correct values for the min / max (the auto range works fine). I added a PR (#1326) that allows adding "dtype": "source" to the style at the top level (e.g., {"bands": [...], "dtype": "source"} that will return the histogram in the original data type.

Not necessarily for this PR:

Minor styling desires: Can we make "Max Merge" fit on the same line to reduce vertical height slightly?

The label for reference with max-merge quite do what you'd expect. Specifically, if for property of a label contains the id of another element, then clicking on the label acts on that element. Since this references the name property, it doesn't allow clicking on the words to activate the checkbox.

Otherwise, the only issue I see is one of performance -- we ask for the histograms faster than we can handle them resulting in having stale requests when scrubbing quickly. Can we debounce that in any way?