Creates new Layer.
@param {Object} [config] An object with config options.
@param {String/HTMLElement} [existingEl] Uses an existing DOM element.
If the element is not found it creates it.
function(config, existingEl) {
        config = config || {};
        var me = this,
            dh = Ext.DomHelper,
            cp = config.parentEl,
            pel = cp ? Ext.getDom(cp) : document.body,
            hm = config.hideMode,
            cls = Ext.baseCSSPrefix + (config.fixed && !(Ext.isIE6 || Ext.isIEQuirks) ? 'fixed-layer' : 'layer');

        // set an "el" property that references "this".  This allows
        // Ext.util.Positionable methods to operate on this.el.dom since it
        // gets mixed into both Element and Component
        me.el = me;

        if (existingEl) {
            me.dom = Ext.getDom(existingEl);
        }
        if (!me.dom) {
            me.dom = dh.append(pel, config.dh || {
                tag: 'div',
                cls: cls // primarily to give el 'position:absolute' or, if fixed, 'position:fixed'
            });
        } else {
            me.addCls(cls);
            if (!me.dom.parentNode) {
                pel.appendChild(me.dom);
            }
        }

        if (config.id) {
            me.id = me.dom.id = config.id;
        } else {
            me.id = Ext.id(me.dom);
        }

        Ext.Element.addToCache(me);

        if (config.cls) {
            me.addCls(config.cls);
        }
        me.constrain = config.constrain !== false;

        // Allow Components to pass their hide mode down to the Layer if they are floating.
        // Otherwise, allow useDisplay to override the default hiding method which is visibility.
        // TODO: Have ExtJS's Element implement visibilityMode by using classes as in Mobile.
        if (hm) {
            me.setVisibilityMode(Ext.Element[hm.toUpperCase()]);
            if (me.visibilityMode == Ext.Element.ASCLASS) {
                me.visibilityCls = config.visibilityCls;
            }
        } else if (config.useDisplay) {
            me.setVisibilityMode(Ext.Element.DISPLAY);
        } else {
            me.setVisibilityMode(Ext.Element.VISIBILITY);
        }

        if (config.shadow) {
            me.shadowOffset = config.shadowOffset || 4;
            me.shadow = new Ext.Shadow({
                offset: me.shadowOffset,
                mode: config.shadow,
                fixed: config.fixed
            });
            me.disableShadow();
        } else {
            me.shadowOffset = 0;
        }
        me.useShim = config.shim !== false && Ext.useShims;
        if (config.hidden === true) {
            me.hide();
        } else {
            me.show();
        }
    }