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.