Added "visbility" to conditions https://github.com/MetaModels/core/issues/587
Merge pull request #597 from tim-bec/condition-label

Added "visbility" to conditions description text to clearify #587.
Add "autoload-dev" section to composer.json.

This allows to reuse the unit tests and drop the hardcoded loading of
classes in bootstrap.php.
New palette condition to determine if a property within a model is a MetaModel.
Merge branch 'tng' into rework-sorting