Integrate icalendar.prop.image and icalendar.prop.conference into property types

[niccokunzmann]

The wrapper classes can be made obsolete if the Interface of all the value types is the same.

#658 showcases that we can create just return the value types and unify the parameters.
For the case of conference and image, please

• evaluate suitable creation methods (like new_image or new_conference or just new)
• create usage & creation documentation
• copy documentation over into the places where it is useful
• maybe link to getters/setters of components from the prop.py file