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)
Minor index fix, for entries with spaces
Cleaning up debugging code
Fix typos in strings
E-book viewer: Add a keyboard shortcut to bookmark current location. Fixes #1326198 [Enhancement to Reader– single keystroke bookmark.](https://bugs.launchpad.net/calibre/+bug/1326198)
Merge branch 'master' of https://github.com/pgarst/calibre
Check Book: Fix combination of mismatching font family names and a font-family declaration with an invalid value (such as a dimension) causing auto fixing of errors to fail. Fixes #1325070 [Private bug](https://bugs.launchpad.net/calibre/+bug/1325070)
Check Book: Fix empty (zero byte) XML files causing check book to fail. Fixes #1326073 [Private bug](https://bugs.launchpad.net/calibre/+bug/1326073)
Edit book: Fix changing underline style to none when creating a custom color scheme causes error.