Add interfaces inheritance support

[ilyawaisman]

If @StrFlag'ged-interface extends another interface that interface should also be @StrFlag'ged (and confine corresponding requirements). Flags representation should be generated respecting inherited flags. Parent flags shout go first. If an interface extends several interfaces, order parent flags should reflect the order of their specification in extends clause (if Java reflection gives here some guarantees otherwise we have need to invent something).