Use relaxed validation for element templates

[nikku]

Is your feature request related to a problem? Please describe

In the properties panel we currently have two validation modes:

• A strict mode, only applying for critical properties (ID)
• A lax mode, where invalid user contents will be accepted, but indicated through linting

In strict mode we do not serialized changed values with the goal to maintain the integrity of the model. Applying it in the context of element templates leads to unpleasant surprises (#831).

Describe the solution you'd like

• Validate element template bindings in relaxed mode
• Keep error indication on the panel

Describe alternatives you've considered

Keep existing validation in place.

Additional context

None.

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Related to #831

[nikku]

CC @christian-konrad