Implement game logic

[Azuelle]

In game_logic.h and .cpp, implement the actual gameplay logics and processing happening each cycle, following the description in README.

What needs to be processed

• Check if item timers have run out
  • Execute onDeath() afterwards
  • If item is bomb
    • Check if player's in range
      • Modify Health
    • Check if other items are in range
      • Execute their onDeath() and delete afterwards
      • Watch out for repeated executions (probably should have a flag for executed ones)
  • Remove item
• Respond to player action
  • Validation
  • Change position / add item / etc. if valid
• Item generation (maybe not each frame)

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As this would be a pretty big task, we should probably have multiple people work on this (or split this into smaller tasks).