(graphics): add default visibility configuration for SLandmarks adaptor

Description

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

Closes #635 (closed)

How to test it?

• Build and launch an application using SLandmarks adaptor.
• In the configuration xml file, add visible="false" to the SLandmarks and check that the landmarks are hidden by default.
• You can also add a button to toggle visibility of landmarks to check that it works fine.
• Or use the attached patch: mypatch.patch

Data

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