On book merge, merge identifiers as well. In case of conflict, the target wins.
Merge branch 'merge-identifiers' of https://github.com/hakan42/calibre

When auto-merging books on add, also merge identifiers.
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Conversion: Add option to embed all referenced fonts

Conversion: Add an option to embed all fonts that are referenced in the
input document but are not already embedded. This will search your
system for the referenced font, and if found, the font will be embedded.
Only works if the output format supports font embedding (for example:
EPUB or AZW3).
AZW3 Input: Add support for page-progression-direction

AZW3 Input: Add support for the page-progression-direction that is used
to indicate page turns should happen from right to left. The attribute
is passed into EPUB when converting. Fixes #1194766 [Incorrect conversion japanese MOBI](https://bugs.launchpad.net/calibre/+bug/1194766)
PDF Output: Fix add ToC option not being used

PDF Output: Fix Table of Contents being added tot he end of the PDF even
without the Add Table of Contents option being enabled. Fixes #1194836 [When convert to PDF, it always create TOC at the end](https://bugs.launchpad.net/calibre/+bug/1194836)
Merge remote-tracking branch 'upstream/master' into gshome