Start work on plugin framework for editor
Fix Documentation for list_re, list_re_group, first_matching_cmp, transliterate.

Fixes #1339296 [Fix documentation for some formatter functions.](https://bugs.launchpad.net/calibre/+bug/1339296)
Fix some font directories from fonts.conf being ignored on linux when scanning the system for installed fonts. Fixes #1339257 [some font directories are ignored](https://bugs.launchpad.net/calibre/+bug/1339257)
Edit Book: Fix inline spell check context menu not working when triggered by context menu key or a right clicking at the right edge of the word.
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Edit book/Book polishing: Preserve the class attribute on the <html> tag in AZW3 format files. Fixes #1339055 [text layout changed from vertical to horizontal when using "Polish Books" feature](https://bugs.launchpad.net/calibre/+bug/1339055)
Edit Book/Book polishing: Do not auto-resolve @import rules in AZW3 files. Also do not add a @namespace rule to all stylesheets that contain tag selectors.
Edit Book: Highlight href/src attributes and allow clicking on them while holding down the Ctrl key to jump to the linked to file
Also highlight URLs in CSS files and make them Ctrl-clickable