Add support for auxiliary signals in NX Image visualization

[axelboc]

Fix #1500

I considered many implementation and UI options but eventually settled with the simplest I could think of:

• In terms of implementation:
  • I worked mostly inside NxImageContainer -- I didn't change anything in DimensionMapper, MappedHeatmapVis, HeatmapToolbar, HeatmapVis, etc.
  • The UI is rendered in the same container as the container (i.e. I didn't use a portal).
  • I added a single local state to the container, which is not persisted.
• In terms of UI/UX, I went with a very basic group of radio buttons just above the visualization. Can't do any simpler than that!

[axelboc]

/approve

[axelboc]

I now wonder if we should not use this radio button UI for Phase/Amplitude selection for complex datasets instead of the dropdown ?

Interesting thought. The current Amp/Phase drop-down is indeed an exception in the toolbar: it controls the what instead of the how like other controls. I'll take a note to revisit this at a later time.

[loichuder]

it controls the what instead of the how like other controls.

Exactly :+1:

[axelboc]

/approve