Further refactor ModelProperty

[jsnel]

The PR #914 brought some much needed refactoring issue to light, as @s-weigand pointed out:

But also if works quite well, for 0.6.0 we should really reflect on if we really need this kind of metaprogramming or if we could maybe solve our requirements in a more maintainable fashion and pythonic way, instead of our mad scientist (e.g. utilizing attrs).

For me the most problematic point is inheriting a from the property decorator class, which leads this strange tests in test_model_property that don't reflect the actual usage.
The more tests the married, but they should reflect the usage in the code.

For now, there is no quick way around it (at least after 2h of trial and error from my side) and it would be out of scope for this PR, since this is an integral part of the code for 3 years.

Originally posted by @s-weigand in #914 (review)