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