Compared to the recent PR 15722 for the `textLayer` this one should be a (comparatively) much a smaller win overall, since most documents don't have any structTree-data and the required parsing should be cheaper. However, it seems to me that it cannot hurt to improve this nonetheless.
Note that by moving the `structTreeLayer` initialization we remove the need for the "textlayerrendered" event listener, which thus simplifies the code a little bit.
Also, removes the API-caching of the structTree-data since this was basically done to offset the lack of caching in the viewer.
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