Gullwing IPC: inherit from FootprintGenerator

Inherit the gullwing generator from FootprintGenerator (the SMD generator was the first one)

This allows:

• Adds the -o parameter to output directly to a directory
• Simplifies a bit of copypasta
• Provides access to the type-safe GlobalConfig

It can't quite fully replace the mushed-into-a-dict global config just yet due to the 'references' structure, but it can do the rest.

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To test (there are zero changes in output files over master):

• First regenerate with master
• Copy the output to the FP repo
• Commit that
• Switch to this branch
• Regenerate (now you can use -o to outut directly, no copy required)
• Observe that there is no diff