Since Qt5.7, there has been a new version of the QtQuick Controls: https://doc.qt.io/qt-5.11/qtquickcontrols2-differences.html
The version 2 should be faster and take less resources.
We are actually using the version 1.
All QtQuick 1 item don't have an equivalent in QtQuick 2 but we need to find on how to update to QtQuick 2.
Note that we can't commit this as long as we have Qt5.6 as minimal requirement.
All the changes to apply can be found in https://invent.kde.org/education/gcompris/-/commit/e01e7531546837e29ebb0439b11fa2376c05bac7:
[] Button -> GCButton
[] ExclusiveGroup -> ButtonGroup (in Categorisation/PlayPiano)
[] Qt: 5.6.0 -> 5.7.0 in CMakeLists.txt
[] Ship good plugin in src/core/CMakeLists.txt
[] Slider: minimumValue/maximumValue ->from/to, scrollEnabled -> wheelEnabled
[] ProgressBar: minimumValue/maximumValue ->from/to
[] TextField: textColor -> color, remove style (Menu.qml)
Missing:
[] StackView
[] Calendar