Fixed a strange bug in the QGraphicsView with an alternative implementation.

- If the player started a new game with the same board size, some lasers on the right and on the bottom were not visible (!). The player had to resize a bit the window to let them display -> they were here, but just not displayed... Origin of this bug is still not clear...

- Anyway: Now, the lasers are hidden and not destroyed so that we can "recycle" them with the start of a new game with the same board size. (With a different board size, lasers are still destroyed and recreated.) With this implementation, the bug disappears. :)