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
Remove frames around diff text views so that the diff lines cross the separator cleanly
Edit Book: Add check for links that point to directories in the book instead of files. Fixes #1272769 [Check Book in Edit Book end with Unhandled exception](https://bugs.launchpad.net/calibre/+bug/1272769)
Fix keyboard shortcuts not working for diff view when in dialog