Visibility/Validity attribute istrue and isfalse for validation against boolean properties. #100
Description
Is your feature request related to a problem? Please describe.
There is no direct way of validating a menu based on a property which is set to a boolean value. For example, based on #90, the property selection.dir.empty could be used.
Describe the solution you'd like
A new attribute for Visibility / Validity nodes should be create to allow validation against boolean properties. Suggested names for these new properties are istrue, isfalse. The inverse attribute should also support these new attributes.
Describe alternatives you've considered
You can define expression and use exprtk attribute but that is not very intuitive and user friendly.
Additional context
This is a follow up on #90.