fmbt-editor: fixed disappearing AAL/Python lines issue

Added a workaround to something that looks a QSyntaxHighlighter +
QTextEdit issue: some (but not all) lines without any formatting by
highlightBlock becase invisible in QTextEdit. Adding some formatting
on every line helped.
fmbt-editor: --preview-only mode added

running "fmbt-editor --preview-only model.aal test.conf" enables
editing model and test configuration files with your favourite editor
and using fmbt-editor only for visualising the model, test and parsing
errors.