It was agreed that we should write "plug-in" consistently. Only possibly user-visible strings were updated. Thanks to scootergrisen for a first patch which could not make it after changing decision on the canonical writing.
This fixes many typos in comments and one in a user-visible string (msgid "center abscisse" changed to "center abscissa" in affected po files. too).
This only renames files, no code changes.
