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Even if we dont need account for omemo, its easier to supply always
the same count of arguments to all encrypt_file() plugin methods.

[OMEMO] Make some strings translatable

See merge request !54

Now we dont need to supply the account

- Make Pillow the fallback loader
- Simplify loading of images
- Use Gtk.Image instead of Gajims TextViewImage

As in contacts not in our roster and contacts we are not fully
subscribed to.

- Dont depend on Gajim method, as it triggers Events that will be
shown in the chat window