Adapt code to GenerateProperties=true in CalendarSettings

This change the return type the <Type>Item from
KConfigSkeletonGenericItem<Type> to KConfigCompilerSignallingItem.

Which are similar but not quite the same (the
KConfigCompilerSignallingItem is a wrapper around the GenericItem)
so add an overload for that in addWidBool.

See akonadi-calendar!60


(cherry picked from commit 04ff7956)