add viewBox

Signed-off-by: Michael Leuschel <leuschel@uni-duesseldorf.de>

Button/button.svg

-<svg height="240" width="200">
+<svg id = "root" 
+   viewBox = "0 0 200 240",
+   width = "200",
+   height = "240"
+>
   <circle id="button" cx="100" cy="100" r="80"
     stroke="black" stroke-width="3" fill="green" />
     <text  text-align="left"  x="50"  y="190"