new KIPI ImageInfo wrapper to easy manage item properties with KIPI host application.

This class will prevent to patch libkipi in the future to be able to manage more properties,
_WITHOUT_ to break binary compatibility between plugin and host application.
The goal is to always use KIPI::*attributes*() methods to exange information between plugin and host.
This wrapper will be a collection of methods to help developpers to items get/set properties without complixity.
It can be extended without any problem and without to touch libkipi.
Currently only description properties is managed, others lead item properties will be supported later...
CCMAIL: kde-imaging@kde.org
CCMAIL: digikam-devel@kde.org