Pretty print view hierarchy

See merge request redox-os/orbtk!99

Added a depth parameter to the expand function to support pretty-printing the view hierarchy in debug mode.

Example:
```
| MainView (id = 1, children_len = 1)
| | Row (id = 2, children_len = 1)
| | | Column (id = 3, children_len = 5)
| | | | Container (id = 4, children_len = 1)
| | | | | TextBlock (id = 5, children_len = 0)
| | | | Container (id = 6, children_len = 1)
| | | | | Button (id = 7, children_len = 1)
```

fix typo

See merge request redox-os/orbtk!97