Added setting controls for image layers.

[Carifio24]

This PR implements the functionality that was discussed at this morning's glue meeting - adding controls for image layer settings. See the image below for a visual depiction of the layer options widget. Additionally, a cmap attribute is added to the image layer class, which is wired up to the cmap attribute of the underlying WWT layer.

The code changes here are relatively minimal since most of the wiring between the glue and pywwt layers is already in place; they just needed to be exposed to the Qt application. @pkgw, if you have any thoughts on any changes/other things to add here, that would be great.

[codecov[bot]]

Codecov Report

Merging #88 (683d78d) into main (608d879) will decrease coverage by 0.51%. The diff coverage is 30.76%.

@@            Coverage Diff             @@
##             main      #88      +/-   ##
==========================================
- Coverage   71.97%   71.46%   -0.52%     
==========================================
  Files          18       18              
  Lines         860      869       +9     
==========================================
+ Hits          619      621       +2     
- Misses        241      248       +7     

Impacted Files	Coverage Δ
glue_wwt/viewer/jupyter_viewer.py	0.00% <0.00%> (ø)
glue_wwt/viewer/image_layer.py	33.00% <28.57%> (-0.67%)	:arrow_down:
glue_wwt/viewer/image_style_editor.py	63.63% <50.00%> (-11.37%)	:arrow_down:

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[pkgw]

I'm really not familiar with the glue side of things, but superficially this looks reasonable!

[Carifio24]

I've also added a commit to connect the new cmap property of the image layer state to the (pre-existing) layer controls in the Jupyter viewer.