Avoid unnecessary dynamic_casts, LayerObject base class, remove CollisionListener

[Vankata453]

Unnecessary dynamic_casts have been replaced with static_casts, or the code requiring them has been improved.

Add LayerObject class

LayerObject is a base class for all GameObjects which are shown under the EditorLayersWidget. Used to prevent multiple casts every single frame for every single layer icon when determining z-pos.

Closes #701.

Remove CollisionListener class

The changes done by commit fa82f91 have been reverted. The reason is that CollisionListener was not used as a base anywhere beside MovingObject, and made it so CollisionObject::collide()/collision() had to use dynamic_cast to side-cast the other object's listener to a GameObject.

CollisionObject now contains the MovingObject& m_parent member, as opposed to CollisionListener& m_listener.

Additionally, the now base MovingObject::collision... functions now accept MovingObject& in arguments, instead of GameObject&. This saves on a few other casts along the way.

[tobbi]

I'm gonna test this in a few.

[tobbi]

This also fixes #701

[tobbi]

In file included from /Volumes/Development/workspace/supertux/src/badguy/scrystallo.cpp:21:
/Volumes/Development/workspace/supertux/src/object/player.hpp:198:7: warning: 'get_coins' overrides a member function but is not marked 'override' [-Winconsistent-missing-override]
  int get_coins() const;
      ^
/Volumes/Development/workspace/supertux/src/supertux/game_object.hpp:167:15: note: overridden virtual function is here
  virtual int get_coins() const { return 0; }
              ^
In file included from /Volumes/Development/workspace/supertux/src/badguy/short_fuse.cpp:23:
/Volumes/Development/workspace/supertux/src/object/player.hpp:198:7: warning: 'get_coins' overrides a member function but is not marked 'override' [-Winconsistent-missing-override]
  int get_coins() const;
      ^
/Volumes/Development/workspace/supertux/src/supertux/game_object.hpp:167:15: note: overridden virtual function is here
  virtual int get_coins() const { return 0; }
              ^
In file included from /Volumes/Development/workspace/supertux/src/badguy/root_sapling.cpp:23:
/Volumes/Development/workspace/supertux/src/object/player.hpp:198:7: warning: 'get_coins' overrides a member function but is not marked 'override' [-Winconsistent-missing-override]
  int get_coins() const;
      ^
/Volumes/Development/workspace/supertux/src/supertux/game_object.hpp:167:15: note: overridden virtual function is here
  virtual int get_coins() const { return 0; }
              ^
In file included from /Volumes/Development/workspace/supertux/src/badguy/skydive.cpp:21:
/Volumes/Development/workspace/supertux/src/object/player.hpp:198:7: warning: 'get_coins' overrides a member function but is not marked 'override' [-Winconsistent-missing-override]
  int get_coins() const;
      ^
/Volumes/Development/workspace/supertux/src/supertux/game_object.hpp:167:15: note: overridden virtual function is here
  virtual int get_coins() const { return 0; }
              ^

[tobbi]

I tested it, seems fine. I couldn't notice much of a benefit, only thing I might have noticed is bonus block hitting seems to be way less laggy (there used to be a slight lag before).

[swagtoy]

Dynamic cast used in the collision code??????????????????? Good lord. This PR might bring about a substantial performance boost O_O

Best it gets merge asap to prevent any future blockers in other PRs?

[swagtoy]

std::move me. Compiler probably wouldn't care anyway.

[swagtoy]

Nothing else to say, just decide on std::move. lgmt :)