Improve the *istD pad

[Michel Carrère-Gée]

I stuck to the center pad a transparent plastic stop; it is a 
transparent adhesive round, diameter 8mm, thickness 2mm.

The plastic does not slip under the finger and the pad is more handy, as 
a joystick one pushes towards the direction desired instead of pushing.

http://perso.wanadoo.fr/krg/temp/pad-istd.jpg
Michel