ADD: new track message, Entity class and Position class

libs/eigen/demos/opengl/README

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+
+Navigation:
+ left button:           rotate around the target
+ middle button:         zoom
+ left button + ctrl     quake rotate (rotate around camera position)
+ middle button + ctrl   walk (progress along camera's z direction)
+ left button:           pan (translate in the XY camera's plane)
+
+R : move the camera to initial position
+A : start/stop animation
+C : clear the animation
+G : add a key frame
+