Interface for read-only objects.
This interface is JSFML's way to map C++ const references to Java. Subinterfaces will
be specific to a certain JSFML type and provide methods to read from them only.
The subinterfaces may be used freely outside of the JSFML API itself, however, it is not
recommended to implement them, because in certain cases, they are expected to be implemented
by a certain JSFML class.