Update last_mod times when custom column metadata changes instead of triggering a backup.
Avoid re-writing all OPF files on custom column metadata change

Merge branch 'master' of https://github.com/cbhaley/calibre
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Phys Org by Krittika Goyal
PDF Output: Serialize ToC string as UTF-16

PDF Output: Fix non-breaking space characters incorrectly rendered in
PDF outline. Fixes #1223862 [PDF TOC -  &nbsp; appears sometimes as €](https://bugs.launchpad.net/calibre/+bug/1223862)
Driver for PocketBook Mini 515

Fixes #1223853 [Pocketbook 515 recognition](https://bugs.launchpad.net/calibre/+bug/1223853)
Book polishing: Apply null comments

Book polishing: Do not leave behind the old comments when updating
metadata if the comments have been deleted in calibre.