Clarify box creation options
The latest mockups include some changes to the box creation workflow: https://gitlab.gnome.org/Teams/Design/app-mockups/blob/master/boxes/boxes-newbox-assistant.png
Specific changes:
- Separate dialogs are used for adding VMs, remote connections and brokers
- When adding a VM, source selection is split into sections with headers: detected sources, featured downloads, "Select a source"
The goal of these changes is to clarify each of the options when creating a box. There's some more detailed background in Teams/Design/app-mockups#9 (closed) .