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Start work on API documentation for the new database API
More API documentation
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API documentation for the Container class
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Move all China related recipes into a single category

Fixes #1338001 [Change in categorization of news sources](https://bugs.launchpad.net/calibre/+bug/1338001)
More API documentation
When calculating the recommended folder for files of a given type, if no such files already exist in the book, use the folder containing the OPF file rather than the root
Book polishing: When changing the cover in an EPUB file, place the newly created cover image in the same folder as other images in the book, rather than the root
Finish API documentation for the container and associated tools
Port compiling of .ui forms
Port imports
Port build script for building PyQt extensions to PyQt5
Port the progress indicator extension to PyQt5
Allow controlling PyQt5 import path via the env var PYQT5_IMPORT_PATH
faster builds
Get rid of the PyQt4 string from everywhere
Port: setMargin() reset() QStringList()

setMargin(): QLayout no longer has a setMargin() function, replace with
setContentsMargins()

reset(): QAbstractListModel no longer has reset(), replace with
beginResetModel(), endResetModel()

QStringList: No longer exists, a simply python list of strings works
instead
Fix mouse wheel not working in diff view