Lightroom round-trip: re-import client-approved selections

[Luca-Timo]

Plugin-side implementation of the picpeak feature request PicPeak/picpeak#745.

Scope: round-trip client-approved selections from picpeak galleries back into Lightroom — re-import the picks/flags/ratings the client made in the gallery as a collection or flags on the matching catalog photos.

Parent: PicPeak/picpeak#745