Separated the grid detection and registration in an extra thread so that the kinects can run at 30fps and the time difference between the used point clouds is smaller.

- Using only the centers of the grid (in case it is not ensured that each Kinect sees the grid from the same side).
- With all the centers of the holes of the grid for faster registration, if it is ensured that the grid is always seen from the same side (e.g. Kinects on the ceiling and look down).

Currently, the images in the sample project are not synchronized (two threads, there can be a delay of multiple images between both kinects!), so the grid must move extremely slowly as it is currently implemented. This will be fixed soon.