Monitors may not always have a mode. This means we cannot look up the
mode on the fly, but instead need to have a heuristic to select a mode
if there is none.
Everything was in place already for this, except that one location was
still doing a fresh lookup, possibly causing a crash.
Fixes: #675
This means we show the first 5 scales that are supported for the
display. That is the same behaviour as 3.30 had, therefore minimizing
the UI changes that users will see.
Note that there are plans to improve the scaling UI, however, it is not
yet clear how this will look like.
