Index of the button

 ~raw 1 -> left button 2 -> center button 3 -> right button  ~

Property definitions

sdl2 $ SDLMouseButtonEvent :: button
	# Index of the button
	#
	# ~~~raw
	# 1 -> left button
	# 2 -> center button
	# 3 -> right button
	# ~~~
	fun button: Int `{ return self->button.button; `}
lib/sdl2/events.nit:204,2--211,50