plug-ins: clean all warnings on metadata plug-in.
I made the various static arrays in headers declared as extern and defined in a separate implementation file, with additional size constant when necessary (since G_N_ELEMENTS() cannot be used on partially defined arrays). Note that this is better than the original implementation, yet I am not saying this is ideal either. I am not fond of such code organization and think a better one could be done. But it would require to dive deeper into this plug-in and I don't want to right now, nor have the time. At least now it stopped shouting all these warnings!
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