Add a test for property notification

Add a test for property notification

This tests just a few basic things for now. Mainly, that we don't
emit redundant notifications for enum, flags, int and boolean
properties. It also checks that we do emit the expected notifications
when the value actually changes. This is checked for string, double
and float properties as well.

There is a large number of exceptions in the test, and a lot more
checks that could be done. One class of exceptions is all the places
where we have -set booleans to go along with another property. We
should have a dedicated test for these pairs. Another class of
exceptions is where naked objects created by g_object_new () just
don't have the full functionality - e.g. a tree selection without
a tree view does not work very well. We set up the instance object
better for these situations.