Add full support for keySig in layer

[rettinghaus]

In some prints (mid-measure) clef changes are followed by an additional key "reminder":

Trying to encode this leads to a key signature rendered at the wrong position (before the clef), with the correct "new" position of the accidentals:

Click to show MEI example

```xml

7 4+ 2+ 6 6⃥

```

It would be nice to see full support for keySig. related to #2792