At the moment, a platform should provide two output lists - one that
lists all available outputs, and the other one that contains only
enabled outputs. In general, this amounts to some boilerplate code and
forces backends to be implemented in some certain way, which sometimes
is inconvenient, e.g. if an output is disabled or enabled, it will be
simpler if we only change Output::isEnabled(), otherwise we need to
start accounting for corner cases such as the order in which
Output::isEnabled() and Platform::enabledOutputs() are changed, etc.
