Support images in tooltips (for TSLD)

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I requested if we could add the functionality to see the structures in the tooltip similar to DecisionSite.

To be able to implement this, the TS visuals need to first support adding images to the tooltip, preferably via the existing CustomValueRenderer extension point.

API-wise there should also be some way of synchronizing renderer choice and renderer settings between table plots and tooltip.

To be fully functional, there must also be a mechanism like the "virtual columns" that delay-loads the data needed for rendering. Maybe the new data-on-demand could be used, since TSLD only supports information links for virtual columns anyway, but I have no idea what the 3.1 data-on-demand is so I could be totally wrong.

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