Allow retrieving nyan::Object handle as shared_ptr

[heinezen]

This allows fetching a nyan::Object from a nyan::View as std::shared_ptr.

Even though

nyan/nyan/object.h

Lines 63 to 67 in 19e742a

	// This constructor is public, but can't be invoked since the Restricted
	// class is not available. We use this to be able to invoke make_shared
	// within this class, but not outside of it.
	Object(Object::Restricted, const fqon_t &name, const std::shared_ptr<View> &origin) :
	Object(name, origin) {};

is in the code, I'm not sure why nyan::Object handles shouldn't be shared. I might be missing something there, but I don't see the disadvantage of enabling the shared pointers.

The main motivation for this change is a change to the activity system in openage PR SFTtech/openage#1688 to include nyan::Objects. Using raw objects/references makes testing the activity system more complicated as it requires us to initialize a whole nyan database for the tests. If we use shared pointers, we can get away with nullptr wrapped in shared_ptr as a workaround.