Adapt to change of KStandardAction::name(id) return type
Actually with KF5 all the code passing on the return type of KStandardAction::name(id) to KActionCollection::action(name) would need to convert from const char* to QString, while in KF6 both use QString, so no conversion is needed.
Just, kdenlive uses auto-cast, and all but one of places of such code rely on that, do not do an explicit conversion. So the most simple fix seems to just align that one place :)