Add crossobject collision

[pkulev]

Closes issue(s): [ #52 ]

Description of the changes:

• What's new:

  • Game experience. Enemies damage player, enemies die from damage. GameOverState shows your total score.

• Fixes:

• Refactoring:

  • Refactor collision.py module to collect handlers on import time
  • Move Collider<->CollisionManager link to State<->CollisionManager (more appropriate for System)

TODO:

• Fix tests
• Fix unduying colliders in case of killing enemy (when ship lefts screen all is ok)

[codecov-io]

Codecov Report

Merging #66 into master will decrease coverage by 2.34%. The diff coverage is 50.4%.

@@            Coverage Diff            @@
##           master     #66      +/-   ##
=========================================
- Coverage   61.95%   59.6%   -2.35%     
=========================================
  Files          39      40       +1     
  Lines        2142    2280     +138     
=========================================
+ Hits         1327    1359      +32     
- Misses        815     921     +106

Impacted Files	Coverage Δ
xoinvader/ship.py	0% <0%> (ø)	:arrow_up:
xoinvader/ingame.py	0% <0%> (ø)	:arrow_up:
xoinvader/game.py	0% <0%> (ø)	:arrow_up:
xoinvader/menu.py	0% <0%> (ø)	:arrow_up:
xoinvader/gui.py	0% <0%> (ø)	:arrow_up:
xoinvader/keys.py	100% <100%> (ø)	:arrow_up:
xoinvader/tests/test_collision.py	100% <100%> (+1.12%)	:arrow_up:
xoinvader/state.py	100% <100%> (ø)	:arrow_up:
xoinvader/charge.py	50.84% <46.15%> (-2.35%)	:arrow_down:
xoinvader/render.py	45.45% <50%> (+0.29%)	:arrow_up:
... and 4 more

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