KConfigSkeletonItem : allow to set a KconfigGroup to read and write items in nested groups

Summary:
Currently KConfgiSkeleton cannot manage item entry in subgroup, like

[General][Colors]
MyItem=foo

Example of use

// Generated Class with KConfig compiler, inherits KConfigSkeleton

KConfigGroup generalGroup( &config, "General" );
KConfigGroup colorsGroup = config.group( "Colors" )
myItem->setGroup(cg); // Now can take a KConfigGroup
addItem(myItem, "MyItem");

Evolution of kconfig compiler will follow to consider this.

Reviewers: ervin, dfaure, Frameworks, mdawson

Reviewed By: ervin, dfaure

Subscribers: kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D27059