Group participants by parent in plantuml structurizr seq diagrams

[Leooo]

It is quite hard currently to understand large sequence diagrams created using Structurizr, as the participants are not grouped by parent.

This PR is starting to resolve this. It may also be a first step towards adding more advanced sequence diagram features relevant to C4 in the future, e.g. allowing to collapse or expand sequence diagram participants using element parents.

• [x] group components in Plantuml structurizr sequence diagrams by parent containers and by grand-parent software systems.

• [x] group containers in Plantuml structurizr sequence diagrams by parent software systems.

• [x] group elements in Plantuml structurizr sequence diagrams by parent groups.

• [x] add plantuml.teoz property to allow nested boxes in sequence diagrams - see teoz layout engine details here

• [x] styling of parent and grand-parents on sequence diagrams uses what is defined in styles object

First example (components grouped by containers then softwareSystem):

After changes in this PR - the elements with the same parent (f, g and h) are now grouped while they were not before. Both parent and grand-parent elements are shown. Any group is also shown

Second example (containers grouped by software systems):

After changes in this PR - the parents Software Systems of the container participants are shown:

Third example

After changes - elements belonging to the same container are grouped:

Possible extensions / improvements to this PR - to be discussed

• add the changes proposed in this PR under a property flag to be backward compatible.
• ensure that, inside a given parent box, the order of steps mirrors the order they were declared (in the first example above the participant f is not the first anymore).