Add (C4 styled) Sequence Diagram support

[kirchsth]

related to #307

In the final solutions the sequence diagram supports descriptions, line breaks, links, ... of all elements.
The only main difference are
a) boundaries have to be defined without { and } and instead of } the Boundary_End() macro has to be called
b) descriptions of the elements (person, system,... ) are deactivated; they can be activated via the layout option SHOW_ELEMENT_DESCRIPTIONS(?show)
c) in contrast to a normal sequence diagram the foot boxes are deactivated; they can be activated via SHOW_FOOT_BOXES(?show)

The C4 language itself is basically not extended, missing new features like Grouping messages have to added with native PlantUML syntax.

I have no useful sample at the moment, maybe someone can attach a more practical sample

(part of the new docu)

(C4 styled) Sequence diagram

C4-PlantUML does not offer a full sequence diagram support,
but existing elements and relationships can be reused as participants and calls in the corresponding styles.

!!! Contrary to all other diagrams, please define boundaries without { and } and mark a boundary end with Boundary_End() !!!

• Import: !include https://raw.githubusercontent.com/kirchsth/C4-PlantUML/extended/C4_Sequence.puml

• Additional Macros (based on component diagram macros):

  • Basically all element specific macros (Person, System, Container...) can be reused with following differences:
    • element descriptions are typically not displayed (can be activated via SHOW_ELEMENT_DESCRIPTIONS())
    • boundaries have to be defined without { and } and instead of } the Boundary_End() macro has to be called
  • Additional (element specific) Macros:
    • Boundary_End()
  • Additional (element specific) Layout Options:
    • SHOW_ELEMENT_DESCRIPTIONS(?show)
    • SHOW_FOOT_BOXES(?show)
  • Only following 3 relationship specific macros are supported:
    • Rel($from, $to, $label, $techn="", $descr="", $sprite="", $tags="", $link="")
    • RelIndex($e_index, $from, $to, $label, $techn="", $descr="", $sprite="", $tags="", $link="")
    • RelSpecialIndex($rel, $e_index, $from, $to, $label, $techn="", $descr="", $sprite="", $tags="", $link=""):
      $rel enables the definition of all PlantUML specific arrow types, details see e.g.
      All arrow types and
      Slanted or odd arrows
  • The 5 index related macros (like the dynamic diagram)
    • (lowercase) increment($offset=1): increase current index (procedure which has no direct output)
    • (lowercase) setIndex($new_index): set the new index (procedure which has no direct output)
    • LastIndex(): return the last used index (function which can be used as argument)
    • Index($offset=1): returns current index and calculates next index (function which can be used as argument)
    • SetIndex($new_index): returns new set index and calculates next index (function which can be used as argument)

• (Typically additional used) PlantUML statements:

  • Grouping message
  • Divider or separator
  • Reference
  • Delay

(end of the new docu)

Themes supports sequence diagrams too (details see new section in Themes.md)

It can be tested via my extended branch

BR Helmut