Preferences->Metadata plugboards: Show plugboards for disabled devices and formats whose metadata writer plugins have been disabled. Fixes #1360766 [display plugboards with disabled plugin](https://bugs.launchpad.net/calibre/+bug/1360766)

Merge branch 'master' of https://github.com/cbhaley/calibre
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OS X: Fix background jobs stealing input focus if they require the use of hidden browser
Proper OS X command line tools path
Fix #1360806: allow book.smart_update to copy "null" (Unknown) author information to the destination if the destination author and author_sort values are null. There is a small performance hit if the source of the copy is null, which fortunately is rather rare. This change better supports non-null author_sort values associated with null author values.