applet: Correct horizontal delta values

When swiping fingers on touchpad from left to right, one expects that
the volume will increase but it's the opposite right now.

The applet accumulates delta values. If the accumulated value reaches
120, volume will be increased; if it reaches -120, volume will be
decreased.

Scroll up and right produce deltas with different signs. This change
flips delta values of horizontal scroll events so scroll right events
produce delta values with the same sign as scroll up events.