Refactor sandbox code so that a JVM entrypoint can be made.

[Kietyo]

This needs to be submitted first before the fix for the grayscale issue will be fixed.

[codecov-commenter]

Codecov Report

Patch coverage: 70.06% and project coverage change: +0.06 🎉

Comparison is base (2543f7b) 51.16% compared to head (bc54b50) 51.22%.

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Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1810      +/-   ##
==========================================
+ Coverage   51.16%   51.22%   +0.06%     
==========================================
  Files        1744     1747       +3     
  Lines      102477   102605     +128     
  Branches    14481    14493      +12     
==========================================
+ Hits        52428    52562     +134     
- Misses      46116    46120       +4     
+ Partials     3933     3923      -10     

Flag	Coverage Δ
unittests	51.22% <70.06%> (+0.06%)	⬆️

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Impacted Files	Coverage Δ
...sandbox/src/commonMain/kotlin/samples/MainCache.kt	0.00% <0.00%> (ø)
...korlibs/korge/view/StandardViewPropertyProvider.kt	0.00% <0.00%> (ø)
...mMain/kotlin/korlibs/korge/awt/UiEditProperties.kt	0.00% <0.00%> (ø)
...nMain/kotlin/korlibs/korge/view/CachedContainer.kt	76.54% <62.50%> (-3.74%)	⬇️
...src/commonMain/kotlin/korlibs/io/util/NumberExt.kt	81.69% <75.00%> (+7.84%)	⬆️
...rim/src/commonMain/kotlin/korlibs/image/Quality.kt	92.30% <92.30%> (ø)
...a/src/commonMain/kotlin/korlibs/math/geom/Scale.kt	84.61% <94.73%> (+65.56%)	⬆️
.../src/commonMain/kotlin/korlibs/korge/view/Views.kt	64.59% <100.00%> (+0.11%)	⬆️
...src/commonMain/kotlin/korlibs/render/GameWindow.kt	39.85% <100.00%> (+0.97%)	⬆️
...src/commonTest/kotlin/korlibs/image/QualityTest.kt	100.00% <100.00%> (ø)
... and 1 more

... and 20 files with indirect coverage changes

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