RichTextArea renderer polish

[koppor]

Follow-ups for the rich renderer (split from #2, where the adoption evaluation and implementation are documented):

• RTF/HTML clipboard export via RichTextArea's built-in format handlers
• Scroll-to-search-hit navigation via TextPos (JabRef: jump the preview to the first <mark> highlight)
• Semantic theming variables: ship the .rich-text-area structural rules in html-to-node.css against library-defined looked-up colors, so consumers set palette variables instead of skin-internal selectors (interim until the upstream styleable property lands)
• Hand cursor / underline-on-hover over links

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