Check Book: When reporting unmanifested file warnings, allow auto fixing by adding the file to the manifest if it is referenced elsewhere or removing the file if it is not. Fixes #1323362 [Automatic fix does not work for "file is not listed in the book manifest."](https://bugs.launchpad.net/calibre/+bug/1323362)
Merge branch 'master' of https://github.com/pgarst/calibre
Start work on custom theme support for the editor
Preview panel for theme customization dialog
Allow removal of custom themes
Edit Book: Allow creation of custom color schemes for the editor (Preferences->Editor settings)
Add edit book action to main toolbar by default
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Fix #1324162 [Authors gives   error: unhandled exception  /  IndexError:string index out of range](https://bugs.launchpad.net/calibre/+bug/1324162)
version 1.39.0
Update to support Kobo firmware 3.4.1

Kobo are about to release firmware 3.4.1. The beta and release
candidates have been tested with calibre and the version number is all
that needs to change. As usual, I am bumping it to 3.4.2 in case of a
second bug fix release with no backend changes. Fixes #1324375 [Update to support Kobo firmware 3.4.1](https://bugs.launchpad.net/calibre/+bug/1324375)
pep8
Fix #1324433 [calibre, version 1.39.0 FEHLER: Problem beim Starten des wireless Gerätes: Der Wireless Treiber hat Probleme beim Starten. Problem: "start smartdevice unknown exception"](https://bugs.launchpad.net/calibre/+bug/1324433)