[#157] added keyboard shortcuts to the platformer example allowing to adjust global volume
[#157] added keyboard shortcuts to the platformer example allowing to adjust global volume

(test case)

rlugojr/melonJS,gopalindians/melonJS,srbala/melonJS,gopalindians/melonJS,melonjs/melonJS,TukekeSoft/melonJS,TukekeSoft/melonJS,melonjs/melonJS,MarkoIvanetic/melonJS,rlugojr/melonJS,gopalindians/melonJS,MarkoIvanetic/melonJS,rlugojr/melonJS,srbala/melonJS,srbala/melonJS,TukekeSoft/melonJS