Show borders between levels

[alexsb]

In the table, neighbors and parents can be right next to each other. We should have different line width in the table frames to separate rows of different levels.

For example,

Level 1 
==========
Level 2 A
--------------
Level 2 B
--------------
Level 2 C
==========
Level 3 A
==========
Level 2 D

[cnobre]

though not initially intended to mark this separation, the hierarchy bars serve this purpose as well.