Now the user can add directional lights to the 3D views.
I added a tab widget under the lights section of the 3D configuration dialog.
The user can specify the direction of the light, its color and intensity.
This is just a duplication of the already existing point lights implementation with some changes.
Later it may be useful to add a way to visualize the light direction by displaying a mesh that is positioned at camera's view point like an arrow that displays how the sun lights the scene for example.