Add visualisation mode for landmarks observations

[almarouk]

Linked to alicevision/QtAliceVision#42

• add an option to visualize the landmarks observations
• observed landmarks for current camera + links to cameras observing the same landmarks
• linked to the 2D viewer to zoom on specific landmarks

[natowi]

It would be great if an edit mode could be added on top of this feature in a following contribution :)

[simogasp]

That's very interesting!
Another interesting feature would be colour-coding the 3D points based on their visibility (i.e., the number of observations). That gives a hint of how "reliable" the 3D point is.