Rename cmap_mode in image viewer to color_mode for consistency

[astrofrog]

This is testing out a new feature added in echo to be able to have aliases for callback properties. For now this just changes the property without changing all the instances of using cmap_mode to check that the alias does indeed work.

The 2D scatter mode and glue-wwt both use color_mode, and I think that name makes more sense since the choice is between fixed color and cmap.

cc @Carifio24

[astrofrog]

@Carifio24 - does glue-ar depend on this property? If so, could you test this PR out with the latest developer version of echo and glue-ar to make sure everything is working well on that side?

[Carifio24]

@astrofrog glue-ar is only concerned with the 3D viewers, so it won't be affected by this unless the 3D scatter state inherits from this one.

glue-plotly probably does though, so I could use that for testing this

[astrofrog]

@Carifio24 - that'd be great, could you confirm if glue-ar works fine with this?

[Carifio24]

@astrofrog I ran the glue-plotly test suite with this PR and dev echo, and it passed fine! So at that level I think this is okay

However, I also tried opening up glue-qt in the same environment. The application opens fine, but the layer options panel doesn't open. The glue terminal gives an error about the C++ combo box being deleted, but the issue ultimately comes from here

I can confirm that if I replace that line with (kinda gross, maybe there's a cleaner check?)

type_prop = getattr(type(instance), prop)
        if not (isinstance(type_prop, SelectionCallbackProperty) or (isinstance(type_prop, CallbackPropertyAlias) and isinstance(type_prop._target_property, SelectionCallbackProperty))):

then the panel will open, so we might need to make checks of this sort in the echo connection classes.

[astrofrog]

@Carifio24 I think the Qt issue will be fixed in echo 0.12.1 (building right now)