Fix possible abort when creating incidences with tags

When a new incidence was created with a tag, a call chain ran from
`IncidenceChanger::CreatedFinished()` through
`ItemEditorPrivate::itemFetchResult()` to `IncidenceCategories::load()`
which did nothing; then `CombinedIncidenceEditor::load(Incidence::Ptr &)`
would Q_ASSERT because the categories editor was still dirty.