Checkstyle fixes.
Tweaking menu screens to improve layout.
Added a method to Color for adding it to a byte buffer.
Further tweak to column layout, each widget is given the entire available row height for rendering.
Fixed an NPE with UIImage when no image is set.
Fixed some issues when rendering elements with no space.
Added PreviewWorldScreen.
OpenGLTexture now reuses its existing texture id during reload.
Merge branch 'develop' into nui

Conflicts:
	engine/src/main/java/org/terasology/math/Vector2i.java
	engine/src/main/java/org/terasology/rendering/gui/widgets/UIText.java
Checkstyle fixes