Add PBD::Timing::elapsed_msecs() as convenience API

libs/pbd/pbd/timing.h

@@ -93,11 +93,16 @@ public:
 		return elapsed;
 	}
 
-	/// Elapsed time in microseconds
+	/// @return Elapsed time in microseconds
 	uint64_t elapsed () const {
 		return m_last_val - m_start_val;
 	}
 
+	/// @return Elapsed time in milliseconds
+	uint64_t elapsed_msecs () const {
+		return elapsed () / 1000;
+	}
+
 private:
 
 	uint64_t m_start_val;