Changes to Layers Pane and MouseEvents in jDrawingArea

MouseEvents now draw immediately using graphics to the current
BufferedImage and then to the JPanel for mouseClicks and MouseDrags as
well as store them as SimpleHistory objects. The Layers Pane is now more
responsive to user input and properly displays multiple layers.

***Currently a bug where when changing the selected layers, they won't
display until the next action on the JPanel. Zach suggested either a
custom JPanel for jDrawingArea or perhaps a way to override
paintComponent directly at the source of repainting.