Note how the end of the `{PDFOutlineViewer, PDFAttachmentViewer, PDFLayerViewer}.render` methods share *almost* identical code, hence we can reduce some duplication by introducing the new `BaseTreeViewer` helper method here.
Furthermore, setting `this._lastToggleIsShow` can be made ever so slightly more efficient, since we don't care about the number of ".treeItemsHidden"-classes but only want to know if at least one exists.
These two classes are unsurprisingly quite similar, and with upcoming changes[1] the amount of (essentially) duplicated code will increase even further.
Notable changes:
- Collect shared functionality in the `BaseTreeViewer` class, reducing both current and future code-duplication.
- Reduce unnecessary duplication in the CSS rules, which will be particularly useful with upcoming changes.
- Tweak the attachmentsView to use links, rather than buttons, to simplify (primarily) the CSS rules.
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[1] Once API support for "Optional Content" lands, I've got more-or-less finished patches to add viewer support as well.
