Use inspect.getdoc when populating _builtin_type_docstrings

IPython.core.oinspect.getdoc uses inspect.getdoc which
strips some whitespace internally and because of that

    inspect.getdoc(property) != property.__doc__
Add property-specific oinspect tests

Test that:

* property fget/fset sources are returned
* if property method is built-in, its qualname is printed
* property docstring is taken from fget and looked up