Implement the From trait for some simple conversations between Clipboard
related structs.

This PR only adds the From trait implementations and doesn't touch any
code. In a future PR we can simplify usage throughout the codebase, such
as:

```rust
// impl ClipboardString
fn new(text: String) -> Self {
    Self::from(text)
}
```

Release Notes:

- N/A *or* Added/Fixed/Improved ...