"Unused" state can not be removed from item

[Anders Melander]

Steps:

1. Add a resourcestring to a Delphi project.
2. Create a translation project and translate the resourcestring.
3. Remove the resourcestring from the Delphi project.
4. Update the translation project: The translation and resourcestring is marked “unused”.
5. Add the resourcestring again.
6. Update the translation project.

Expected:

The translation and resourcestring is no longer marked “unused”.

Actual:

The translation and resourcestring is still marked “unused”

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[Anders Melander]

Fixes #17: "Unused" state is now cleared if items reappear in project.

→ <<cset f69012053ffa>>