(SPointList): add slots to update and toggle visibility

What does this MR do?

• Add a toggleVisibility slot in SPointList
• Implement this feature in ExOpenvslam

How to test it?

• Build and launch ExOpenvslam
• Load the given data following this MR
• You can now show/hide openvslam pointcloud with the given dedicated toolbar button

Data

• https://owncloud.ircad.fr/index.php/f/1772490

Additional tests to run

• Build on Linux