+/** @brief Compare two tracks
+ * @param sa First sort key
+ * @param sb Second sort key
+ * @param da First display string
+ * @param db Second display string
+ * @param ta First raw track
+ * @param tb Second raw track
+ * @return -ve, 0 or +ve for a <, = or > b
+ *
+ * Tries the following comparisons until a difference is found:
+ * - case-independent comparison of sort keys
+ * - case-dependent comparison of sort keys
+ * - case-independent comparison of display strings
+ * - case-dependent comparison of display strings
+ * - case-dependent comparison of paths (see compare_path())
+ */