Merge the PaintBrush class into the BasePaintBrush class.

- The context, stroke colour, and image surface are now passed into the
DoMouseMove method. This removes the dependency on the PaintBrushTool
class and allows the code to move into Pinta.Core.

- This should make writing addins with custom brushes a bit simpler, as
classes can just inherit from BasePaintBrush.