make the viewer interruptible

[gdementen]

As a follow-up to #237... I thought it was impossible, but I found by accident some crazy code in matplotlib making it possible, so we can now fix the situation properly.

It would use a private method of matplotlib, so we will need to either live with the fact it can break at any matplotlib upgrade or vendor in the code.

from matplotlib.backends.qt_compat import _maybe_allow_interrupt

with _maybe_allow_interrupt(qt_app):
    qt_app.exec_()

[gdementen]

It broke faster than I thought 😉. The code mentioned above worked for matplotlib 3.8 but no longer works for 3.9+. The function was moved and renamed. I guess vendoring the code is the safest option.