This adds a variable mu4e-action-tags-completion-list, that contains a
list of commonly used tags to suggest as completion terms during a retag
actions.
Along the way, the retag action accepts as argument a comma-separated
list of +tag and -tag keywords, instead of a space-separated one,
removing the need to quote tags with spaces in them, and making it
consistent with the behaviour of completing-read-multiple.
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