Support smooth scrolling brightness change

[matzipan]

After #57 is closed, the indicator will finally have scroll on the indicator icon. However, neither the icon scroll neither the scroller scroll can handle smooth scrolling gracefully. Currently, #57 increments the brightness 10% for every scroll event, which means a small amount of smooth scroll can lead to a big change.

To support smooth scrolling, the handler needs to check if the event is smooth and then change the brightness value with a value scaled by the delta_y

[peteruithoven]

This should be implemented using a similar approach as the sound indicator, see: handle_scroll.
(since PR: elementary/wingpanel-indicator-sound#81)

[peteruithoven]

Looks like this is an issue on behavior of a rejected PR, let's close this an put all the scroll related requests in: #20