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- 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

- PEP is not reliable enough for now, so we have to use this as
fallback

- create a OMEMOConnection class to mimic more how Gajim does things

HistoryWindow doesnt have a ChatControl

- In minimized MUCs we dont have a parent window
- use Gtks get_active_window() instead

- This is needed so url_image_preview works

Limit image preview to https and httpupload

See merge request !47