Allow specifying only the tab name in the `tabs` section
- For example this is now possible:
```
tabs:
- name: first
parts:
- direction: Vertical
- direction: Vertical
- name: second
- name: third
```
For that the tab section defaults the direction to
`direction::Horizontal`
- Adds an error upon specifying a tab name inside the `parts` section
of the tab-layout
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| Cargo.toml | ||