Make camera tracking configurable from startup

[chapulina]

When we migrated camera tracking functionality from GzScene3D to CameraTracking, we forgot to port the ability of configuring it at startup.

Desired behavior

Camera tracking is configurable from XML.

Alternatives considered

N/A

Implementation suggestion

Here's the code that needs to be ported:

https://github.com/ignitionrobotics/ign-gazebo/blob/fc2a98ec5e1b149d773eeb6cae4a407bd82a81a2/src/gui/plugins/scene3d/Scene3D.cc#L2726-L2770

Additional context

Camera tracking added in #226

[GauravKumar9920]

can I work on this issue ?

[nick4fake]

How can this be achieved with the current state?