Improve equalitycheck of DependencyObject.PropertyChanged

When changing DepdendencyProperties there is an equalitycheck to prevent unecessary events when the value is actually the same.

However, this is based on the Type of the DependencyProperty and uses `Object.Equals` for string and valuetypes and `Object.ReferenceEquals` for eveything else.

The problem is, that certain things fail the EqualityCheck that should actually not.

My suggestion is to extend this check to respect `IEquateable` and/or `IEquateable` to prevent unecessary changed events.