Integrating that in our CI is currently a bit hard, so disable it for
now, if GStreamer isn't found. Just make sure to build against GStreamer
for call support!

While it does improve performance a bit, it is simply bad for most
distributions, since they don't know and probably don't want to
recompile Nheko. So default it to off. Anyone who wants it can still
toggle it on and it is almost no maintenance cost.

May fix #213

This is necessary to support having a picker within QML.
Eventually, this should replace the existing widget-based one.

closes #94

Fixes #166

fixes #124