Replace static_plugins.h by CMake `qt5_import_plugins()`
See https://doc.qt.io/qt-5/qtcore-cmake-qt5-import-plugins.html
We need to wait until we've got Qt 5.14 everywhere (CI) though
Edited by Linus Jahn
See https://doc.qt.io/qt-5/qtcore-cmake-qt5-import-plugins.html