[3647] Add support for diagram style with background customization

[frouene]

Bug: #3647

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[sbegaudeau]

No need to make it optional, most of our APIs are there to fail fast, we are asking for a style, we should receive one

[sbegaudeau]

color="transparent"

[pcdavid]

Any reason to have some of the DiagramStyle in the tests with the default (transparent) background and others with .bkacground("#FAFAFA") like the one below in the same file?

[frouene]

No, in fact, I started by adding a value at instantiation and then adding a default value.
I changed to use the default value in the tests

[sbegaudeau]

I still have a couple of things to review but this first point is an important one because it could give a wrong impression for downstream consumers (they may believe that this code works).

[sbegaudeau]

This must be new since the last time I reviewed this PR because this cannot work. I could accept that in the services of Papaya diagrams because it would work in this case but imagine that I have 1 node in my diagram which has 10 child nodes and the view deletion request is for one of its child node then this code does not work.

Imagine now that I have 3 nodes in my diagram and a tool triggers the creation of 3 view deletion request then this code would not work either. Finding out that a diagram is empty in all use case is more complex than what you algorithm suggest. Please move both the implementation of this service and its definition out of IDiagramServices for now and into something in the Java services of Papaya (since I suspect that you are only using it there).

You could also improve this algorithm to make it work all the time but it will require some testing of corner cases. Consumers of Sirius Web such as SysON would be quite interested by such service.