Hidden attributes

- clean code by removing all *attribute_() methods
- GraphListeners is now in org.graphstream.util and is common for AbstractGraph and GraphicGraph
- add support for hidden attributes, for attribute name with a leading dot
- add few things to improve performances when dealing with ui attributes : there was a lot of call to the equals method to check attribute name that can be globally conditionnaly removed by checking the two first characters of the attribute name

No change in the public API.