i3bar: implement custom mouse wheel commands

i3bar: implement custom mouse wheel commands

Users can specify a command to run when a button was pressed on i3bar to
override the default behavior. Currently only the mouse wheel buttons
are supported. This is useful for disabling the scroll wheel action or
running scripts that implement custom behavior for these buttons.

Example:

bar {
    wheel_up_cmd nop
    wheel_down_cmd exec ~/.i3/scripts/custom_wheel_down
}

fixes #1104