Mouse wheel scroll in mixer applet

Here's a hack to put on work the mouse wheel on the volume mixer applet. 

The code was taken following the instructions on [here] (http://comments.gmane.org/gmane.comp.gnome.desktop/46513). 

*Side note*: I cannot compile consort-applets right now ( I'm on SolusOS 1.3 ). Also, when I was on SolusOS2 I cannot get compiled the mixer applet ( don't remember why )