gui: PreviewGeneratorPlainText: Show line numbers

Generate previews that show the line number and surrounding
lines (like grep -C) for context.

gui/previewgeneratorplaintext.h

@@ -1,5 +1,6 @@
 #ifndef PREVIEWGENERATORPLAINTEXT_H
 #define PREVIEWGENERATORPLAINTEXT_H
+#include <QTextStream>
 #include "previewgenerator.h"
 
 class PreviewGeneratorPlainText : public PreviewGenerator
@@ -10,6 +11,7 @@ class PreviewGeneratorPlainText : public PreviewGenerator
   public:
 	using PreviewGenerator::PreviewGenerator;
 	QString generatePreviewText(QString content, RenderConfig config, QString fileName);
+	QString generateLineBasedPreviewText(QTextStream &in, RenderConfig config, QString fileName);
 	QSharedPointer<PreviewResult> generate(RenderConfig config, QString documentPath, unsigned int page);
 };