SystemStorage should inherit from EventTarget

[halton]

Same as

 interface SystemDisplay : EventTarget {
    readonly    attribute DisplayUnit[] displays;
                attribute EventHandler  onconnect;
                attribute EventHandler  ondisconnect;
};

Storage should do same thing

interface SystemStorage : EventTarget {
    readonly    attribute StorageUnit[] storages;
                attribute EventHandler  onattach;
                attribute EventHandler  ondetach;
};