Generates unique ids. If the element already has an id, it is unchanged
@param {HTMLElement/Ext.Element} [el] The element to generate an id for
@param {String} prefix (optional) Id prefix (defaults "ext-gen")
@return {String} The generated Id.
function(el, prefix) {
        var me = this,
            sandboxPrefix = '';
        el = Ext.getDom(el, true) || {};
        if (el === document) {
            el.id = me.documentId;
        }
        else if (el === window) {
            el.id = me.windowId;
        }
        if (!el.id) {
            if (me.isSandboxed) {
                sandboxPrefix = Ext.sandboxName.toLowerCase() + '-';
            }
            el.id = sandboxPrefix + (prefix || "ext-gen") + (++Ext.idSeed);
        }
        return el.id;
    }