Consider removing from file headers the line mentioning the "Frescobaldi Extensions project"

[igneus]

... because it becomes misleading whenever someone bootstraps their new extension by copying the boilerplate. The "Frescobaldi Extensions project" is basically over and it seems to be impossible to become part of it (even if you really want to - and we shouldn't expect extension authors to automatically want that).

ref igneus/frescobaldi-file-browser-extension#2