Non-cryptic way to add spacing in a View or Group

Some of our project code has views that resemble this:

```python
View(
VGroup(
Heading("Some heading"),
HGroup(
"15",
Item("my_interesting_trait"),
"15",
Item("my_other_interesting_trait"),
),
),
)
```

Those `"15"`s are cryptic, and hard to do a doc search for. The meaning is documented under the `Item` docstring in the API docs, but you still have to do some guessing or code diving to even think to look at the `Item` docs.

It would be nice to have a non-cryptic alternative to these, e.g., something like `Spacer(15)` or `Padding(15)`.