Mer/Headset switch events

On Maemo 4, this can be triggered by plugging in the headphone that has been supplied with the N8x0 tablet and then running:

dbus-monitor --system

Pressing the button should result in the following D-Bus signal to be sent:

signal sender=:1.1 -> dest=(null destination) path=/org/freedesktop/Hal/devices/platform_retu_headset_logicaldev_input; interface=org.freedesktop.Hal.Device; member=Condition string "ButtonPressed" string "phone"

This is used by several media players (e.g. Panucci) to support Play/Pause or Next Track from the headphone switch.