Changed childItemTypes from ItemType[] to Array<>
Updated ItemManager for randomization, fixed bugs

I updated the update() method of ItemManager to make it generate new
items. Some bugs popped out such as a NullPointerException during
fadeIn() which is caused by having the Render component's color
uninitialized. I fixed it by initializing color in Render's constructor.
Updated WubGame.draw() to draw specific opacity

By setting the SpriteBatch's Color, we can change the opacity of the
Texture being rendered.
I also changed the values of the Render's default Color from (0, 0, 0,
1) to (1, 1, 1, 1). Leaving rgb values at 0 will give the texture a
black tint.
Fixed item disappearing after having opacity = 1

Bug is caused by opacity going over 1. Somehow, having opacity > 1 sets
the alpha of Color to 0 +.
Added items property in ShaftAction component

items will be the children of ItemManager's gameObject. This is so we
can check for collision later.