Called when a region (actually when any component) is added to the container. The
region is decorated with some helpful properties (isCenter, isHorz, isVert) and its
splitter is added if its "split" property is true.
@private
function (item, index) {
        var me = this,
            placeholderFor = item.placeholderFor,
            region = item.region,
            split,
            hidden;

        me.callParent(arguments);

        if (region) {
            Ext.apply(item, me.regionFlags[region]);

            if (region == 'center') {
                //<debug>
                if (me.centerRegion) {
                    Ext.Error.raise("Cannot have multiple center regions in a BorderLayout.");
                }
                //</debug>
                me.centerRegion = item;
            } else {
                item.collapseDirection = this.collapseDirections[region];
                split = item.split;
                hidden = !!item.hidden;
                if ((item.isHorz || item.isVert) && (split || item.collapseMode == 'mini')) {
                    me.insertSplitter(item, index, hidden || !split);
                }
            }

            if (!item.hasOwnProperty('collapseMode')) {
                item.collapseMode = me.panelCollapseMode;
            }

            if (!item.hasOwnProperty('animCollapse')) {
                if (item.collapseMode != 'placeholder') {
                    // other collapse modes do not animate nicely in a border layout, so
                    // default them to off:
                    item.animCollapse = false;
                } else {
                    item.animCollapse = me.panelCollapseAnimate;
                }
            }
        } else if (placeholderFor) {
            Ext.apply(item, me.regionFlags[placeholderFor.region]);
            item.region = placeholderFor.region;
            item.weight = placeholderFor.weight;
        }
    }