Handle portlets in missing regions on layout change

Problem/Motivation

If a homebox layout is changed and the regions are different from the ones before, where portlets are already placed, we should show the existing portlets in the "Personalize" tab in an extra area, so the user can drop them into the correct regions.

Currently there's no way to get those portlets.
Still it's an edge-case.

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes