Fixed property assignment issues.

mitk::BaseProperty now uses the non-virtual interface (NVI) idiom
for polymorphic assignment. The assignment operator and copy
constructor of subclasses of mitk::BaseProperty should be disabled
and bool Assign(const BaseProperty&) should be implemented instead
(either private or protected).

Assignable(const BaseProperty&) has been removed and
mitk::PropertyList::SetProperty(...) has been simplified.

New unit tests for polymorphic comparison and assignment of properties.