for images with GWCS, show world coords on mouseover outside of bounding box using refdata

[bmorris3]

Follow up to #3483 to address comments by @pllim in https://github.com/spacetelescope/jdaviz/pull/3483#pullrequestreview-2730199866 Closes #3534

This PR enables mouseover world coordinate info in imviz on images with GWCS, even outside the bounding box, when linked by WCS. If the cursor is outside of the bounding box for the selected image layer, the coord info is taken from the reference data layer, which will be defined with FITS WCS as of #3483. No pixel coordinates are shown if the cursor is outside of the bounding box for an image with GWCS.

In the image below, the cursor is where the red "X" sits, which is on the top image layer:

When the cursor is outside the bounds of the top image layer, the world coordinates are shown in gray, without pixel info:

The same is true if you hover over the not-selected image layer...:

...unless you toggle through the layers in the coord info to Layer A, and you'll see:

Change log entry

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[camipacifici]

This makes sense to me, thank you! Minor comment (see screenshot): probably the markers plugin should not give the pixel value when outside the bounding box. The user does not know about the reference layer and seeing numbers consistent with giant pixels does not make a lot of sense. What do you think?