Allow to give user-chosen positions for network-area-diagram text nodes

[So-Fras]

Describe the current behavior

In network-area diagrams, text node positions are:

• else determined relatively to the position of the voltage level node they are linked to;
• or determined by the force layout algorithm.

Users are not able to assign them a fixed position as an input (as it is possible to do for voltage level nodes).

Describe the expected behavior

Users should be able to assign a fixed position to text nodes as an input, similarly to what is done for voltage level nodes (see FixedLayoutFactory for example).

Describe the motivation

Improve the customization of the network-area-diagram layout.

Extra Information

No response