Move renderer-dependent SurfaceView to renderer's PGraphics #64

After merging #61, Android mode allows to use custom renderers, with the limitation that they have to be either a descendant of PGraphicsAndroid2D, PGraphicsOpenGL, PGraphics2D or PGraphics3D. The only reason for this limitation is the hard-coded SketchSurfaceView and SketchSurfaceViewGL classes, now inside PApplet. But as suggested in #49 (comment), one could move out the renderer-specific implementation of SurfaceView into the renderer's PGraphics, and obtain and instance in PApplet.onCreate() using a static factory method with the following signature PGraphics.createSurfaceView(parent, context, width, hight, clazz).