Fix a QML error since the upgrade to PyQt 5.15.0

The PyQt upgrade is not the cause, but it is the first PyQt version that is showing us the warning at the app startup:

Qt Quick Layouts: Detected recursive rearrange. Aborting after two iterations.

Nothing more. Our turn to try to isolate the QML code and fixing it.

Some thoughts:

• According to https://stackoverflow.com/q/62928391/1117028 is is most likely a GridLayout component that is in cause. The warning is coming from QQuickGridLayoutBase::rearrange().
• Using specific debug envars does not help.