severals improvements in the Graph widget, see below.

- improve Graph label placement to prevent antialiasing
- put all the graphics stuff on a Fbo by default, to prevent any antialiasing.
- add a new SmoothLinePlot for drawing line using a custom shader for perfect AA. (require Kivy 1.8.1).
- reorganize the Plot API to an easier implementation of a custom plot drawing.

Signed-off-by: Mathieu Virbel <mat@kivy.org>