Add problem 40 description (damage texts)
Save DamageText position in its own pos field

Instead of passing position to draw method now we must set center
position for damage text in update() method of its owner.
Add DamageTexts class with basic methods
Add Damageable interface, use it in DamageTexts
Make Player class derive from Damageable

Remove unneeded calls to damage_text_ in Player
Instantiate DamageTexts in Game class

Add Player to DamageTexts object inside of runEventLoop, update and draw
DamageTexts with all other objects.