(graphics): add default visibility configuration for SNegato2D

Description

Adds a tag to configure default visibility in SNegato2D.
Enables the possibility to hide the adaptor by default in the xml configuration

Closes #634 (closed)

How to test it?

• Build and launch an application using SNegato2D
• In the configuration xml file, add visible="false" to the SNegato2D and check that the negato is hidden by default.