Add an option to control how completion works when editing authors/tags/series/etc.

By default, when completing items, calibre will show you all the
candidates that start with the text you have already typed. You can
instead have it show all candidates that contain the text you have
already typed. To do this, go to Preferences->Tweaks->Completion mode.
For example, if you type asi it will now match both Asimov and Quasimodo,
whereas the default behavior would match only Asimov. Fixes #1264304 [[Enhancement] search within string when tagging](https://bugs.launchpad.net/calibre/+bug/1264304)
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