Allow nested datamodels

[Dakror]

When trying to split the UI logic into multiple components, i frequently encounter the issue of not being allowed to nest the respective data models. After simply trying out disabling the check and not seeing any immediate issues as well as no unit test failures, i decided to make this change. I changed the previous error to a info message to let the developer know.

I don't (yet) know of any bad side effects, and if more work would be needed to facilitate this change, let me know. I feel like this is very important to be able to do.

[mikke89]

I don't remember if there was a specific reason for the constraint, or if it was just a conservative choice. In any case, I'm happy with making this change if I feel safe that it works.

One issue I see is that Element::SetDataModel won't work correctly if setting the parent data model last. We should look for other potential issues as well.

If we do eventually support this, I think we don't need the info message, since it is suggesting something non-standard or not supported. But it's nice to leave it in until we consider the feature stable.

[Dakror]

It would probably suffice to check if the child in question has its own data model already and if so stop the descent of the parent model propagation. And yeah i agree about the info message. As always i will use this in my project to see if anything explodes :D

[mikke89]

Thanks for the PR, let us know if you encounter any issues with this.