Move SpriteBase and SpriteWindow.

[MOD] Move src/base/sprite.lua to src/sprite/base.lua.
[MOD] Move src/base/window.lua to src/window/base.lua.
Complete rewritting src/scene/*.lua.

[MOD] Rewrite src/scene/*.
[MOD] Move src/base/scene.lua to src/scene/base.lua.
Rename nexus.core.require to nexus.core.import.

[MOD] Rename nexus.core.require to nexus.core.import.
[MOD] Change all the paths starting from "nexus." instead of "src.".