Finalize beta version of component: Gates

[jeroenheijmans]

Gates (as designed in the tutorial machine) have specialized one-way rollover switches to mark the gate as "visited". When all gates from a specific group have been activated you might get a bonus score, followed by a reset of the rollover switches. Let's finalize these for the alpha release. Stuff to think about:

• Correct physics and continuous collision detection on the walls
• Rollover switches should activate lights/markers for them
• Rollover switches should provide scores
• Optionally for alpha release: if all of them have been lit a bonus score is given and the gates 'reset'