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Fixes #552, i.e. an issue with AJAX request from z3c.form inline validation.
Fix tree javascript for inboxes.

An empty js-selector evaluates to true as well. doh.
Merge pull request #567 from 4teamwork/deif-fix-inline-validation-error

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Merge pull request #570 from 4teamwork/deif-fix-inbox-tree

Fix tree javascript for inboxes.
Referenceprefix manager: Made prefix-mappings of already delete objects removable.