In 997630bfb7 the API changed because of LASH and
jack-session support. The level app property was switched from uint8 to string.
However, some externally maintained frontends (claudia, klaudia and festige) use
the 0.3 API.
This changeset restores the 0.3 API and extends it with new functions and signals,
ones that use string instead of uint8 level.