Allow to customize the behaviour of Akonadi::ItemSync:

 1. Allows subsclasses to "plug in" their own sync algorithms by reimplementing
    virtual bool updateItem( const Item &storedItem, Item &newItem )
 2. Allows to specify how much data ItemSync should fetch from the cache
    when building a list of all items for the collection being synchronized,
    by calling fetchScope() or setFetchScope().

Also, rename localItem => storedItem and remoteItem => newItem within updateItem()
to be consistent with its signature

svn path=/trunk/KDE/kdepimlibs/; revision=825557