Fixing a logic bug on the pencil operation's undo generator
Changing the eraser paint operation to support a differentiated mode on 'Replace' blend mode
Renaming a field on the ColorSlider class
Changing BasePencilPaintTool's response to the middle mouse wheel click
Adding Airbrush mode to the Pencil and Spray Paint oeprations, and reflecting that on the interface
Making the first pixel plot on BasePencilPaintTool happen on its DrawPencil abstraction method