Use object-oriented approach for asset resolver

[rayzchen]

This can be used later for asset resolving in projects, or more general asset loading (for example used by the PyUnity Editor and its assets).

[codecov]

Codecov Report

Merging #73 (3ef9292) into develop (fee9190) will increase coverage by 0.36%.
The diff coverage is 76.14%.

@@             Coverage Diff             @@
##           develop      #73      +/-   ##
===========================================
+ Coverage    53.92%   54.28%   +0.36%     
===========================================
  Files           33       33              
  Lines         5089     5143      +54     
===========================================
+ Hits          2744     2792      +48     
- Misses        2345     2351       +6     

Impacted Files	Coverage Δ
pyunity/info.py	0.00% <0.00%> (ø)
pyunity/resources.py	73.78% <74.72%> (+18.88%)	⬆️
pyunity/gui.py	57.76% <100.00%> (ø)
pyunity/loader.py	65.24% <100.00%> (ø)
pyunity/render.py	30.66% <100.00%> (ø)

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