DeviceLauncherSection: updated to use glib::SignalManager and glib::Object
This allow to be more safe; it fixes also some crashes caused by the fact that DeviceLauncherIcon didn't own the GVolume* object. Now it uses glib::Object completely, so we can automagically ref/unref the volume when the icon is created/removed.. Fixes: https://bugs.launchpad.net/bugs/918548, https://bugs.launchpad.net/bugs/977762. Approved by Andrea Azzarone.