ubuntu18.04 python3-qt5 qtgui SIGSEGV

[Christoph Mayer]

Currently the qtgui test is commented out, e.g., here

In order to make it work one needs to add this line to the corresponding Dockerfile:
apt-get install libqwt-qt5-dev

Otherwise, libqwt imports qt4 libraries in addition to qt5 libraries and performing
from gnuradio import qugui
asserts SIGSEV.

Cheers
Christoph