Fixes #8901

Fixes #8902

Fixes #8894

Avatars are now Surfaces, Gtk.Window has no method yet to add Surfaces
as icons.

Transforming them back to pixbufs is not viable because pixbufs cant hold
the scale information, and we have no way to set it.

This was only used with single chat windows, which is a mode that came
out of style and is probably not used often anymore.

Fixes #8895

https://valadoc.org/gtk+-3.0/Gtk.Button.set_use_underline.html

Delete some already ported methods

- use `with` statement and open() instead of os.open()
- execute run() on dialog, otherwise sys.exit() is called immediatly

getTimestamp() generates always a timestring with a resolution of seconds and
this means time gets always rounded down.
Because we use a resolution of .00000 precision for outgoing messages this
can lead to wrongly sorted messages.

Flatpak: bundle plugin installer

See merge request gajim/gajim!207

- use SendAndCallForResponse instead of triggering an Event
- validate iq result even more, it didnt print iq errors before

Add navigation for chat history

See merge request !202

Fixes #8883

Use keyring module to also handle password storage on linux

See merge request gajim/gajim!206

We dont need to use the ::open handler to handle xmpp uris

Fixes #8881

Update flatpak file

See merge request gajim/gajim!204