Added SpritePicture.

[NEW] Added SpritePicture.
[MOD] Changed the behavior of WindowCommand. Now it uses
Input.isKeyTrigger() to determine whether the C button is pressed.
[MOD] Moved res/graphics/systems/splash1.png to
res/graphics/pictures/love.png.
[MOD] Moved res/graphics/systems/press_and_key_to_continue.png to
res/graphics/pictures/press_any_key_to_continue.png.
[ENH] Improved the code quality of SceneTitle.
Added data/databases/systems.lua.

[NEW] Added data/databases/systems.lua.
[NEW] Added Data.getSystem().