Show the search expression for the virtual library in a tooltip when hovering over the tab for the virtual library. Fixes #1291691 [[Enhancement] VL search expression tooltip on hover](https://bugs.launchpad.net/calibre/+bug/1291691)
Refactor HTML syntax highlighter too keep track of tag nesting

This is needed for the eventual implementation of inline spellcheck
Start work on managing dictionaries in calibre
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Add ISO 3166 country codes
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Implement loading of both builtin and user installed dictionaries
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