Use the "Add Space View/Container" modal for the + button of the blueprint tree

[abey79]

What

This PR wires the new "Add SV/Container" modal to the blueprint tree + button when the "Additive workflow" feature flag is enabled.

Now, the new SV or Container is added to the selected container if the selection consist of a single container. Otherwise the SV or Container is added to the root container.

• Closes: Use the new modal with the blueprint panel "+" button #4652
• Follow-up: The "Add SV/Container" modal should specify somewhere where the new item will be added #5011

add_sv_container_bp_tree.mp4

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• I have read and agree to Contributor Guide and the Code of Conduct
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[Wumpf]

nice, very likable! 😄

[Wumpf]

(merged in main and resolved conflict)