Solo for lanes not saved in the project file

State of solo button for lanes is not saved in the project file.

Steps to reproduce:

  1. Create instrument track.
  2. Create two lanes, then solo one of them.
  3. Save the project.
  4. Open the project.

Actual result: Solo button is not enabled.
Expected result: Solo button should be enabled, as it was before the project has been reloaded.