Edit book: Fix cover not being updated from the current cover in the calibre library when using Edit Book. Note that you can turn off metadata updating via Preferences->calibre Integration in Edit Book. Fixes #1272730 [[enhancement] add selected cover to book](https://bugs.launchpad.net/calibre/+bug/1272730)
Start work on diff dialog
Various minor bug fixes
Various fixes for rendering of images in the diff view