query: Coalesce the title and filename to get consistent ordering

This should make tracker order (by title, or by file name if
no title) match the UI order (by "title", which uses the file
name if there is not title).

Closes: #6

src/query.js

@@ -162,7 +162,7 @@ var QueryBuilder = new Lang.Class({
 
             switch (sortBy) {
             case Gd.MainColumns.PRIMARY_TEXT:
-                tailSparql += 'ASC(?title) ASC(?filename)';
+                tailSparql += 'ASC(tracker:coalesce(?title, ?filename))';
                 break;
             case Gd.MainColumns.SECONDARY_TEXT:
                 tailSparql += 'ASC(?author)';