output: in _Progress class rename output to parent

output is a Output class method and using it in _Progress makes the
code more confusing.
output: Make CompositeOutput inherit from list

Instead of defining a list member inside.
Add a "Log" menu entry in Expert Mode

This can be used to display the current execution log.
Fix minor typo
Fix a flake8 warning
Fix pylint issues
Merge pull request #32 from skalkoto/develop

Add Log menu entry in Expert Mode