Stripping unnecessary information for the first time.

Related to #27.

Closes #15.

* https://code.briarproject.org/briar/briar/-/wikis/changelog#anchor-127
* https://code.briarproject.org/briar/briar/-/tags/release-1.2.7
* https://media.dorfbrunnen.eu/briar/jars/009/

Fixes #13

Thanks to libhandy [1], this is adaptive to different screen sizes.
This commit is heavily based on the work done at GNOME Fractal [2]
and GNOME Lollypop [3]. Huge kudos to them.

[1]: https://source.puri.sm/Librem5/libhandy
[2]: https://gitlab.gnome.org/GNOME/fractal/tree/4df16251f405335d3a7007301530d158fdcf774b
[3]: https://gitlab.gnome.org/World/lollypop/tree/1.2.20

Developed by Purism at https://source.puri.sm/Librem5/libhandy

Documentation at https://honk.sigxcpu.org/projects/libhandy/doc/

Sometimes it seems to work. Only in startup classes it can't access
it...

Also fix do_shutdown function.

The quit function was actually never called before. That was the reason
why the Java background process never stopped.