This generates too much debug output

Fixes #10026

GStreamer is an optional package, we can't depend on it beeing imported

Replaced by gtk/dataform.py

This uses the same mechanism originally added for preferences, so that
video display is handled by a GTK+ widget instead of some X11 dark
magic.

This makes the self video display properly (!) during a Jingle call, but
still with random freezes and no remote video being displayed, those
fixes will be for a latter commit.

These were due to Jingle properties existing for both audio and video,
replace them with the obvious dict solution.

ejabberd uses fallback fields for captchas. Don't show them because
we support captchas.

nbxmpp does all the resolving

Runtime check already uses the new version.

pip checks for runtime requirements as defined in setup.cfg automatically.