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