Re: printing "a 2" at the beginning of staves automatically

[Werner LEMBERG]

>> while it is probably not the best way to do it, here is a
>> pragmatical solution to the problem.
> 
> Thanks a lot!  Will try it soon.

Basically, it works great, but there are two problems (tested with
2.23.9).

* It doesn't print 'a 2' at the very beginning.

* Contrary to `\mark`, the 'a 2' should always be horizontally aligned
  to a note column and not to prefatory matter like time or key
  signatures.

The attached image shows the output of

```
{
  \set Staff.staffAnnotation = "a 2" c'1 1 \break 1 1 1 \break 1 1
  \key c\minor \time 2/2 
  \set Staff.staffAnnotation = "a 3" 1 \break 1 
  \set Staff.staffAnnotation = "a 4" \break 1 2
  \set Staff.staffAnnotation = "a 42" 2 1 
  \unset Staff.staffAnnotation \break 1
}
```

Not sure whether this can be easily fixed without setting up a new
grob that doesn't interfere with `RehearsalMark` – sometimes it is
necessary to horizontally shift 'a 2' without influencing the
positioning of rehearsal marks at exactly the same spot.


    Werner