add the ability for the user to override the background hints anddecide
about applets having background or a shadow
A new handle button will be added to manually enable/disable background for
plasmoids that support it
if they don't, the ImmutableBackground flag is set in the hints.
PlasmaCore.ColorScope has been expanded to work more like Kirigami.Theme (which will be replaced by) even if it's oòld behavior still works. This makes the complementary colorscope in applets with shadow actually work (and at the same time gives a clearer porting path for kf6)