fix mouse click on brightness applet

this fixed https://github.com/mate-desktop/mate-power-manager/issues/62
see
https://bugzilla.redhat.com/show_bug.cgi?id=1018915
https://bugs.launchpad.net/ubuntu/+source/gnome-power-manager/+bug/535097
https://bugzilla.gnome.org/show_bug.cgi?id=612399

Thanks to Andrew Lofthouse