[Julia] Major changes and improvments:

  - Drop support for Julia versions lower that 1.0.0
  - Restore 'Path to Julia' and make its workable
  - Improve UX: now all requirements of this backend handeled by JuliaBackend::requirementsFullfilled (and appears in UI)
  - Forward passing errors (stderr) from cantor_juliasserver to Cantor
  - Improve crash handling: now, if server crashes, message about it appears