Fix a crash that was caused by ColorPaletteSingleton being shared among
multiple QQnlEngines. Each engine requires a unique instance of
ColorEditorSingleton.
* Rewrite ColorEditorSingleton to not be a singleton anymore
* Rename ColorEditorSingleton to ColorEditorBackend
* Add meaningful warnings
Task-number: QDS-4728
Change-Id: I1ed3315add33754b41870ad6f43c1365a899102b
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>
Reviewed-by: Thomas Hartmann <thomas.hartmann@qt.io>