This introduces 3 changes in Helper:
 - 3 Helper functions are now exported
 - resourceColor() checks if a color is set in the preferences files. If yes, it uses it. If not, it checks if there is a CollectionColorAttribute in the collection. If yes, it uses it else it call the resourceColor() from prefs (that can create a new color or return an invalid one)
 - setResourceColor() set a color in the preferences files ONLY IF it's not the same as the CollectionColor Attribute.

Bug: 328862

Differential Revision: https://phabricator.kde.org/D7500