Limit FakeApplicationManagerWindow type

This is a task to mitigate the difference between single- and multi-process mode. An ApplicationManagerWindow inherits from a Window in multi-process mode and from an Item in single-process mode. It should be analysed if and how access to certain properties which are only available in one or the other can be prevented. Most prominent for instance is that a Window does not have a parent property, and hence accessing it in single-process mode (from an Item) should be prevented.

Also the ApplicationManagerWindow documentation should be updated to state exactly what properties, functions and signals are save to use and that everything undocumented is undefined behaviour.