Merge Leveldesign and Save-and-Load (branch: main+leveldesign)

[KarylReyne]

• [x] reset persistent GameData on new game start (@EmptyN8)
• [x] decide what to save persistently in GameData (health, progression, etc.) (@EmptyN8)
• [x] copy missing GameObjects (instantiate from prefabs?) to first Level scene
• [x] enemy script -> prefab (@KarylReyne)
• [x] Trigger: collect attract two (@KarylReyne)
• [x] Trigger: death at black hole (@KarylReyne)
• [x] Trigger: entrance to spaceship (@KarylReyne)
• [x] Trigger: spawn enemies in electro asteroid area (@KarylReyne)
• [x] black hole attracts all objects in scene (@RicoKiwi)
• [x] prettify inventory (@ElectricUnit) ...