`\textLengthOn` for `TextSpanner` grobs

[Werner LEMBERG]

Folks,


please have a look at this example

```
{
  \set Score.skipBars = ##t
  \override TextSpanner.bound-details.left.text = "poco a poco tempo I"
  \override TextSpanner.bound-details.right.text = "Tempo I"
  R1*4\startTextSpan |
  R1\stopTextSpan
}
```

What is the equivalent of `\textLengthOn` for `TextSpanner` grobs so
that the multi-measure rest gets properly stretched horizontally?


    Werner