`Box<str>` is an immutable heap-allocated string slice.
This PR makes it more convenient to use them in labels for example.
Before this PR
```rust
let text: Box<str> = "Hello".into();
ui.label(text.into_string());
let text_ref: &Box<str> = &"Hello".into();
ui.label(text_ref.clone().into_string());
// or
ui.label(text_ref.as_ref());
// or
ui.label(&**text_ref);
```
After this PR
```rust
let text: Box<str> = "Hello".into();
ui.label(text);
let text_ref: &Box<str> = &"Hello".into();
ui.label(text_ref);
```
* [x] I have followed the instructions in the PR template
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