Support multiple inheritance of metamodels

[tdegueul]

Multiple inheritance should be supported with a well-defined semantics (e.g. no conflict or left/right-linearization):

metamodel X { ... }
metamodel Y { ... }
metamodel Z { ... }

metamodel A inherits X, Y { ... }
metamodel B inherits Y, Z { ... }
metamodel C inherits A, B { ... }