display: Ensure configuration has the expected type

NOTE: I can't really test the bug directly, however, I am rather sure that this will fix the bug. The worst thing that should happen is that existing bugs become more visible.

In almost all cases, the configuration will be "valid" in the sense that g-c-c can represent it in the UI. However, there are cases like mirroring setups with three monitors that we do not allow.

In case that the user has such a configuration, ensure that the configuration we represent is actually valid according to our expectations. This should not affect normal use cases, but allows users to recover again if the configuration is broken for some reason.

Fixes #383 (closed)