Caching bug with document browser

[amir-zeldes]

If a corpus has two HTML visualizers (with different display names), the loading visualizer1, closing it, then loading visualizer2, will immediately reopen visualizer1 (with no loading time). It seems the visualization result is cached, and since both are HTML visualizers for the same document, the cache wrongly reproduces visualizer1 instead of constructing the new visualizer2.