Set opacity by default in Graphic2d's so it's not undefined when constructed.

#77
Added test to verify that Opacity is set on Graphic2d construction.

#77
Made the Color object Disposable, and added an OnChange event handler that allows for notifications when the color changes.

#30
Modified the Line2d and Shape objects (objects that have color directly) to tie into the colors OnChange event to determine when it changes.

- This allows for users to modify Colors via their R, G, B values and still have the color update on the game screen.

#72
Fixed sample project to tween opacity correctly for the first tween.
Fixed samples layout to use the new SampleController