applet: Make scrolling to change volume more reliable

After scrolling with a touchpad, scrolling with a mouse can be unreliable at times, for example if wheelDelta has -40 as its value scrolling up once with a mouse adds 120 to wheelDelta resulting in 80, which is neither >=120 nor <=-120, meaning that scroll event gets ignored.

So, once we have enough delta to change the volume reset the delta back to 0.

This is another instance of the bug I first found and fixed in plasma-desktop!1457 (merged).