Remove unused css: Prevent failure when book has invalid items marked as stylesheets
Check Book: Add checks for empty HTML/XML/CSS/image files in the book.  Fixes #1329971 [Private bug](https://bugs.launchpad.net/calibre/+bug/1329971)
Fix #1329777 [Translation DE](https://bugs.launchpad.net/calibre/+bug/1329777)
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Change NON NULL to NOT NULL in the sqlite schema
Change the test db to also use the NOT NULL constraint form
oops
Edit book: Remove unused CSS: Add an option to also remove unused class attributes from the HTML

Edit Book: Remove unused CSS: Handle @import rules when checking for unused CSS
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Add empty book: When adding empty book records to the library, add an option to also create empty EPUB files. Useful if you intend to subsequently edit these files with the calibre editor. You can access the add empty books feature by right clicking the Add Book button.
Remove the need for -- when calling calibre-debug --diff
Prevent Content-Transfer-Enconding headers from confusing encoding detection in the diff tool
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Ignore the translations folder entirely since it is now version controlled in a separate github project.
Implement extraction of user manual strings for translation
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String fixes
When generating sphinx documentation from command line usage/options, try to recognize and mark references to option in the help text
Refactor toolbar creation code for eventual user customizability