Add initial BLI_math_time with a 'seconds decompose' function.

Allows to decompose a given amount of seconds into a random set of
days/hours/minutes/seconds/milliseconds values.

Also add matching test.

Differential Revision: https://developer.blender.org/D11581

source/blender/blenlib/BLI_math.h

@@ -70,4 +70,5 @@
 #include "BLI_math_rotation.h"
 #include "BLI_math_solvers.h"
 #include "BLI_math_statistics.h"
+#include "BLI_math_time.h"
 #include "BLI_math_vector.h"