Move the import & export menu from the Plugins to the File menu.

[tinevez]

So that with the 'naked' Mastodon, there are no items in the Plugins menu. Also import / export items are better in the File menu.

After this: make sure that all I&E plugins in other mastodon artifacts go in these menus.

[stefanhahmann]

https://docs.google.com/document/d/1-hC6YApjdD-58bYWdPDR8Pw3eIyOYTaT5Wtzlx9L-FQ/edit

[stefanhahmann]

[tinevez]

• [x] do this in the core -> JY
• [x] do this for plugins in mastodon-pasteur -> JY
• [ ] do this for plugins in mastodon-tomancak -> Mathias

[stefanhahmann]

Cf. #332