private - used by TabPanel too. Works out the next tab to activate when one tab is closed.
function(toClose) {
        var me = this;
        if (toClose.active && me.items.getCount() > 1) {
            return (me.previousTab && me.previousTab !== toClose && !me.previousTab.disabled) ? me.previousTab : (toClose.next('tab[disabled=false]') || toClose.prev('tab[disabled=false]'));
        }
    }