Maps and object system

[y21]

Is your feature request related to a problem? Please describe.
The "game" (which consists of only one gamemode right now) looks empty without "objects".

Describe the solution you'd like
We should add objects that you (may) interact with:

• Brick walls (interacting ends up in hiding behind them which charges a boost)
• Nom-free area (? #54)
• Items (speed, invincibility, ...)
  Items should occur at random positions and shouldn't be fixed; will disappear after time.
  We also need a system for different maps and a map editor. They should be stored in a dedicated folder ("./maps/").
  Maps differ by having fixed objects (like walls) at different positions.

[y21]

"Objects" (walls) have been added. Maps, however, aren't implemented yet.