Use Qt ColorDialog instead of custom one
This replaced the custom colordialog with the standard one provided by qt. Based https://github.com/KDE/kdeplasma-addons/tree/master/applets/colorpicker
The custom one was created because previously (in qt 5) the qt ColorDialog seemed (in my opinion) out of place and too complicated in a qml application. The new version (qt 6) fixes this issue and adds more features.
Note: the Qt ColorDialog is affected by https://bugreports.qt.io/browse/QTBUG-119055 which forced the use of a workaround. This code will need to be slightly adapted once the bug is fixed.