Unify diagram validation

[barmac]

Is your feature request related to a problem? Please describe.

Right now, validation is scattered in the library and implemented on a case-by-case basis. For example, we validate the simple edit values, but don't warn about empty output names, or a missing element name.

Describe the solution you'd like

Have an API for providing validation similar to the properties panel but in the boxed expression modelers (decision table, literal expression etc.).

Describe alternatives you've considered

Keep the current approach.

Additional context

Related to #852 (comment)