Fix basic_service::get_{xyz} overload set

[bkircher]

Momentarily, we have following functions in the "get_calendar" overload set:

    calendar_item get_calendar_item(const item_id& id);

    calendar_item 
    get_calendar_item(const item_id& id, 
                      const std::vector<property_path>& additional_properties);

    std::vector<calendar_item> 
    get_calendar_items(const std::vector<item_id>& ids, base_shape shape);

    std::vector<calendar_item> 
    get_calendar_items(const std::vector<item_id>& ids, base_shape shape, 
                       const std::vector<property_path>& additional_properties);

Missing/remaining issues:

• no way to specify both, additional properties and base_shape
• plural getters are not implemented for contacts, messages, etc.
• <ExtendedProperty/> missing, see Implement item property <ExtendedProperty> #7
• make sure we do not use default parameters (base_shape, mostly)