Fix metatile label saving

[GriffinRichards]

The old process for saving metatile labels goes like this:

• Parse metatile_labels.h
• Find and discard any existing defines for the tilesets being saved
• Build and insert the new labels into the list of defines.
• Regenerate metatile_labels.h, using a regex to determine which tileset each set of defines belongs to

We have a map of metatile labels from when metatile_labels.h was first parsed, so parsing it again is unnecessary. We also don't need to try and determine which tileset each belongs to when saving (we already know at this point). Now we just refresh this map with the defines for the tilesets being saved, then use this map to regenerate metatile_labels.h. This would discard any definitions that don't belong to an existing tileset, so we now save those separately (this has the added benefit of not printing // gTileset_Name comments in this file for non-existent tilesets). Closes #530

Note: Porymap (as before) does not prevent users from naming tilesets in a way that can cause metatile labels to collide (e.g. if we have gTileset_Foo and gTileset_Foo_Bar, the ownership of METATILE_Foo_Bar_Baz is ambiguous).