New container/space view addition workflow

[abey79]

This issue is about completing the design and implementation of the new container/space view addition workflow, including:

Design

• Design the "Add SV/container" modal
• Work out the workflow from "click the add button in blueprint tree" to "adding stuff with the modal in the selection panel"

Implementation

• Container's selection panel should display children (either container or SV)
• Container's selection panel should have an "add" button
• Implement that "Add SV/Container" modal.
• The "add" button in the blueprint tree should forward to this workflow instead of having its own popup menu.

cc @martenbjork