make level viewport scrollable

[mabre]

Investigate putting centerView into a ScrollPane, or maybe put it into another pane whose transform property is changed?

• [x] viewport is scrollable (manual)
• [x] active figure is automatically centered
• [x] viewport follows projectile (TODO what to do about multiple projectiles?)
• [x] auto-scrolling can be disabled
• [x] viewport changes are animated

[Kegny]

As for the multiple Projectiles: For a Banana for example it should be enough to keep the screen centered on the point, where the shards spawned. They shouldn't fly to far away. For Airstrikes you could maybe choose one(like the middle one)? The others also shouldn't go to far away from the one

[mabre]

fixed with e9c2128049e0bc5d1b156f8d1bee5acd445087ae