Secret Base metatile labels are not properly handled

[GriffinRichards]

Metatile labels for all the Secret Base secondary tilesets are defined with the primary tileset name, so Porymap believes they're invalid and erases them. They're defined this way because the games assume the secondary tilesets all have the same layout, so even if they were properly defined for each secondary tileset only one set of defines would be used.

Perhaps Porymap could allow labels from the primary tileset to overflow into the secondary tileset, and if a label exists in both tilesets then the label from the secondary tileset would be used.