Cannot gracefully edit or delete part of a tag

[dustenhubbard]

Problem

Tag text cannot be edited gracefully: there is no clean way to edit or delete part of a tag, so correcting a typo means removing and re-adding the whole tag.

Proposed

Allow in-place editing of an existing tag, including deleting part of the text, from wherever tags are managed.

Part of #66.

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Source: PyReconstruct Wishlist, Object list, "Tagging".

Editing the Tab is still not graceful - not really able to delete part of the tag text if needed.