Impelemented command execution with DBUS

Used Julia embedding. Features: execution, syntax errors,
exceptions, multiline input, printing of the last result.

Julia path is detected automatically.

BUGS and FIXMES:
* Workaround: julia redirect commands user pipes/sockets with
  limited buffer to store data. So on large output writing pipe
  end blocks, and julia server execution is blocked. It's
  better to reimplement this by using concurrent threads to
  run julia command and read from pipe. Now using files as they
  provide unlimited buffer.

Differential Revision: https://phabricator.kde.org/D2006