Fix ItemAccessors with signalling items, add KConfigSkeletonChangeNotifyingItem for KConfigCompilerSignallingItem

The prefix of the name KConfigCompilerSignallingItem is not matching the other class names and possibly was missed to be adapted, as the other name KConfigSkeletonChangeNotifyingItem with a prefix matching the other classes can be found in the API dox instead.

While so far the class was not used in the public API of generated code fixing the generation of code for ItemAccessors=true with signalling items will leak this class onto the consumer side, so better give it a nice name and also move member data to a pimpl class.

See the need to work-around in network/konversation!70 (diffs) where instead of kconfig_compiler generating the code it is for now manually added to a subclass.

@arichardson @ervin @dfaure @meven