[BUG] Problems starting MDANSE_GUI on openSUSE with Wayland

**Description of the error**
The standard installation of MDANSE_GUI from pypi results in a situation where the mdanse_gui script fails to create the main window on an openSUSE Linux machine with Wayland.

**Describe the expected result**
The GUI should start on every platform without extra tuning.

**Describe the actual result**
Apparently, an additional environment variable is needed:
`QT_QPA_PLATFORM=xcb mdanse_gui`
This makes MDANSE_GUI start correctly.

**Suggested fix**
Normally, Qt has a number of plugins it can use, but for MDANSE_GUI it seems that xcb is the only option.

**Additional details**
N/A

Reproduce the failure by running the standard PyPI-installed mdanse_gui on openSUSE with Wayland, then compare it with QT_QPA_PLATFORM=xcb mdanse_gui. The issue names no source files or tests, so trace the mdanse_gui launch path and Qt platform selection from the installed package. Done means the GUI starts on Wayland without extra environment-variable tuning.

Written by the indexing model from the issue text.