Added PropertiesComparer

[manfred-brands]

Fixes #4394

We constantly have to make special Validate methods or implement IEquatable on classes that should not be comparable.
However we want to be able to compare them in NUnit, e.g. to check if serialization roundtrips.

[OsirisTerje]

Looks good.
Could perhaps had an explicit test for the PropertiesComparer, but I do see that it is perhaps sufficiently covered by many of other tests.

[manfred-brands]

Could perhaps had an explicit test for the PropertiesComparer

None of the other Comparers have their own tests ...
I did add a specific new test for a class that doesn't override Equals, which without this comparer fails and now passes.