Live CSS: Show the Matched CSS header even if no rules match
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Edit Book: Check Book: Do not run the rest of the checks if parsing errors are found, to avoid the checkers from raising unhandled errors
Edit Book: When highlighting the tag the cursor is currently inside, if the cursor is inside the definition of an opening tag, highlight that tag rather than its parent.
More robust implementation of current tag detection for Live CSS. Now uses the same logic as matching tag highlighting.
Use the highlighted matching tags for Live CSS as a performance optimization
Fix regression that caused tags to not be sorted when displayed in the book details panel. Fixes #1351622 [tags are not shown alphabetically in Book Details Window](https://bugs.launchpad.net/calibre/+bug/1351622)
Handle null values in mi.tags when formatting the tags field