version 1.21
Fix editing of narrow yes/no columns in the book list not very convenient

Fixes #1272089 [Making True/False columns narrow displaces the drop down list](https://bugs.launchpad.net/calibre/+bug/1272089)
Rename "Sort by popularity" to "Sort by number of books"
Start work on displaying non-text files in the diff view

Binary files are shown as a single line with the file size. Images are
displayed inline. Currently, resizing images based on the view size
needs to be handled.
Edit Book: Show the filename of the currently edited file in the title bar. Fixes #1272543 [Enhancement : Book Editor - show file path](https://bugs.launchpad.net/calibre/+bug/1272543)
Edit Book: Fix extra 1x1 unused GIF image being created when saving and re-opening AZW3 files. Fixes #1272410 [Editing AZW3 book adds 1x1 pixel gray gif each time](https://bugs.launchpad.net/calibre/+bug/1272410)
Get rid of the unnecessary tracking of changes in the split view
Resize images in diff view when the view is resized
Add a right click context menu action to copy selected text
Context menu actions to go to next/previous change
Highlighting the starting line number for changes
RTF Output: Fix failure to convert documents that contain comments with the -- string inside the comment. Fixes #1272588 [Private bug](https://bugs.launchpad.net/calibre/+bug/1272588)
Refactor all the diff code into a single dir
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
Fix non-linear scrolling in the diff view
Fix theme fallback not working
Add diff colors for other color schemes