Syntax highlighting is called too many times

[giordano]

We should probably rationalise the calls to SyntaxHighlighter.syntax_highlight(): when a file is opened, it's highlighted several times. All of them comes from runSwingLater(NammuController.initHighlighting). In particular:

• for an English-only file, two calls come from AtfEditArea.setText, and one from NammuController.openFile
• for a file with Arabic translation, two calls come from AtfEditArea.setText, one from NammuController.openFile, and six from atfAreaAdjustmentListener.adjustmentValueChanged

It's possible that, e.g., NammuController.openFile itself is calling AtfEditArea.setText, I'm counting the calling places