GTK: add a button sending the document by mail instead of exporting to disk

Motivation: Quite often I find myself exporting documents just to attach them to an email. I must go through two file dialogs and remember the temporary location I used. Simple-scan has a functionality that allows to send the freshly scanned document immediately by email, I wanted to copy this functionality.

Tested on linux with evolution.

The use of os.system is just to get a quick prototype, it'll go away eventually.

When I reached this point in the implem, I realized I had to make one plugin per platform for the functionality "fire up the email client with this attachment". This is above my pay grade. But if using a few if os.name == is ok, I can proceed. Google suggests on windows I just have to run os.startfile(f"mailto?attachment={path}").