Edit Book: Run syntax highlighting in the background so that it does not affect responsiveness of the user interface

Technically, the syntax highlighting now does idle processing, that is
it yields after every line of text is highlighted to allow the rest of
the UI to respond to user interaction. This gives an excellent
illusion of running int he background, except for pathological files
with very long lines (such as those that have all their markup on a
single line).
Fix #1333932 [Edit Book: Double-click on a file category raises a error](https://bugs.launchpad.net/calibre/+bug/1333932)
Fix #1333944 [Edit Book: Strange error when deleting style](https://bugs.launchpad.net/calibre/+bug/1333944)
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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
Port coffeescript compiler
Add note about headless Qt QPA plugin
Port WebKit imports and QChar

WebKit has been split up into QtWebKit and QtWebKitWidgets. Change
imports accordingly.

Replace use of QChar since it no longer exists in PyQt5
Port syntax highlighters
note about creating import checker
...
Port usage of QString and various miscellaneous imports that no longer work