function getActiveFilter() {
        var filterSets        = PreferencesManager.get("fileFilters") || [],
            activeFilterIndex = PreferencesManager.getViewState("activeFileFilter"),
            oldFilter         = PreferencesManager.getViewState("search.exclusions") || [],
            activeFilter      = null;

        if (activeFilterIndex === undefined && oldFilter.length) {
            activeFilter = { name: "", patterns: oldFilter };
            activeFilterIndex = _getFilterIndex(filterSets, activeFilter);

            // Migrate the old filter into the new filter storage
            if (activeFilterIndex === -1) {
                activeFilterIndex = filterSets.length;
                filterSets.push(activeFilter);
                PreferencesManager.set("fileFilters", filterSets);
            }
            PreferencesManager.setViewState("activeFileFilter", activeFilterIndex);
        } else if (activeFilterIndex > -1 && activeFilterIndex < filterSets.length) {
            activeFilter = filterSets[activeFilterIndex];
        }

        return activeFilter;
    }
A search filter is an array of one or more glob strings. The filter must be 'compiled' via compile()
before passing to filterPath()/filterFileList().
@return {?{name: string, patterns: Array.<string>}}