output is a Output class method and using it in _Progress makes the
code more confusing.

According to the decision of the GRNET Board of Directors,
switch license to GPLv3.

This commit will be propagated to the release
and master branches based on git flow, and the next
release will be licensed as GPLv3.

If the text was more lines that the height of the active region in the dialog
the 2 last lines were not displayed.

 * Prefix all private methods with _
 * Write missing docstrings
 * Move all public methods above the private ones
 * Add module docstrings where missing
 * Define source code encodings on all modules

Also fix various issues. snf-image-creator-dialog is almost ready.
Some `Help' entries are missing.

Also create a new output class (InfoBoxOutput) to be used when
performing syspreps