AppImage should be genarated on a "old" distribution to be sure that
libc versions are compatible.
However, we need at least ubuntu bionic to have QWebEngine packaged.
Let's prepare our AppDir directory in bionic and generate the appimage
in xenial.
By using a small launcher script, we check that libc versions are
compatible and else print a nice message instead of crash.
See https://github.com/probonopd/linuxdeployqt/issues/340
Matthieu Gautier