Rewrite WindowBase and rename WindowCommand, WindowCommon, and WindowProgressBar.

[MOD] Rewrite WindowBase.
[MOD] Rename WindowCommand.
[MOD] Rename WindowCommon.
[MOD] Rename WindowProgressBar.
[FIX] Fixed disappearing of progress bar.
Rewrite src/base/color.lua.

[MOD] Rewrite Color.
Rewrite src/base/rectangle.lua.

[MOD] Rewrite Rectangle.
[FIX] Fixed the misused metatable in Color.
Rewrite src/base/tone.lua.

[MOD] Rewrite Tone.
Rewrite src/base/sprite.lua.

[MOD] Rewrite SpriteBase.
[DEL] Remove useless member in WindowBase.
Fixed the usage of SpriteBase.

[FIX] Fixed the usage of SpriteBase.
Rewrite src/base/viewport.lua.

[MOD] Rewrite Viewport.

TODO: Resolve loop loading.