textinput: indicate truncation when value exceeds width

Currently, it is not possible to tell whether a textinput's value was truncated:

```
// is actually: abcdefghijklmnopqrstuvwxyz
Letters: abcdef
```

A truncation indicator in the unfocused view would be very useful here:

```
// when focused: do not show ellipses
Letters: abcde█

// when blurred:

// cursor at the beginning
Letters: abcde…

// cursor at end:
Letters: …vwxyz

// cursor in the middle:
Letters: …ghij…
```