button translation in german

This releates to:
https://bugs.mojang.com/browse/MCPE-12143

"Button"(wooden) is called in german "Knopf" but it should be "Holzknopf".
"Button"(stone) is called in german "Schalter" but it should be "Steinknopf" or at least both buttons should be called "Knopf" like on pc.