Do not create duplicate tags

Merged Glen Ditchfield requested to merge gditchfield/incidenceeditor:create-tag into release/21.08

If an event with categories is cut & pasted, the pasted event has the same categories as the original but does not have the same tag-item relationships in Akonadi. (This is not the only way that categories and tags go out of sync.)

When the incidence editor opens the pasted event, it creates new GENERIC tags for the categories, duplicating the existing tag. Then it inserts the tag in the editor, thus making it dirty and causing unnecessary "Do you really want to cancel?" dialogs.

This patch creates PLAIN tags, merges them with existing tags, and does not dirty the editor.

BUG: 441846

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