PDDL Model Validation (multiple)

[jan-dolejsi]

Today, the extension can be configured with just one validator. The ICAPS 2020 Industrial Session called for a more sophisticated model validation (or perhaps planner-specific model validation).
The Validator concept should be reworked the same way the Planners are now configurable. It should be possible to have multiple validators. But rather than switching between them (radio box), the user should be able to toggle them on/off (check box).