Add a DOI check function to metadata
Add dc:identifier check for DOI & doi validation
...
Check doi in usual info fields
Merge branch 'pdf-meta-check' of https://github.com/sengian/calibre

Look for recognizable identifiers (ISBN/DOI) in the Info dict and
dc:identifier fields, just in case.
Gosc Niedzelny by Piotr Kontek, Tomasz Długosz

Merge branch 'master' of https://github.com/t3d/calibre
Edit Book: Allow typing arbitrary unicode characters with the keyboard in the editor

To use this feature, just type the unicode code number and press Alt+X.
For example, to get the character ÿ type ff and press Alt+X. Or to get
the character 🐱 type 1f431 and press Alt+X

Fixes #1280868 [Windows Alt+ key input does not accept hex digits A-F](https://bugs.launchpad.net/calibre/+bug/1280868)
Edit book: Workaround for bug in Qt that caused extra null bytes to be appended to the end of the file when saving if the file contains non-BMP unicode characters.
Document the Alt+X technique to type unicode symbols with the keyboard
ratings for PDF metadata
Series for PDF metadata
Explicitly clear all known fields when merging XMP packets
title and author sort for PDF metadata
Edit book: Workaround bug in Qt causing crash on deleting bottom most file in the Files Browser on linux
Add languages to PDF metadata
Performance bug: Unneccessary copying of user metadata dict for every field when reading OPF in inner loop
Custom column metadata for PDF
Finish up the rest of the calibre specific metadata for PDF
Workaround for bug in lxml https://bugs.launchpad.net/lxml/+bug/1281139

This bug likely impacts lots of other code locations, but hopefully it
will be fixed in lxml soonish. For the moment, I will add workarounds as
and when I discover they are needed.
HTML Input: Fix incorrect parsing of file names in href/src attributes that contain percent encoded non-ascii characters. Fixes #1281099 [No images/pictures while html to epub  conversion since update to 1.24 !!](https://bugs.launchpad.net/calibre/+bug/1281099)