Handle start/end/merge/decision... graphical nodes on Action Flow View diagram background

[AxelRICHARD]

On Action Flow View diagrams created on ActionUsages (or its subtypes) , ActionDefinitions (or its subtypes), it is not currently possible to create start/end/merge/decision... graphical nodes on the diagram background.

The goal is to be able to create start/end/merge/decision... graphical nodes on the diagram background for such cases.

[jerome-obeo]

I will take care of this issue