Set View Direction icons don't show direction

The Set View Directions -Z and +Z don't show the direction of the Z axis. While you can see that both -Z and +Z are parallel with the viewing direction (perpendicular on the screen) you don't know which is pointing toward the user and which is pointing toward the screen.

This can be fixed by having the X and Y axes overlap the Z axis when Z is pointing toward that screen (+Z).

The same issue is true for -X, +X and -Y, +Y icons.