Bug#1031277: gcompris doesn't start - Your root item has to be a window

[Stefan Kropp <stefan.kropp@posteo.de>]

Got support from the upstream project.

Installing qml-module-qtquick-controls2 qtdeclarative5-private-dev
solved the problem:

apt install qml-module-qtquick-controls2 qtdeclarative5-private-dev

I looks the missing package qml-module-qtquick-controls2 was the
problem. Should qml-module-qtquick-controls2 be added as dependency?

Thanks to the upstream project!