gui: Begin support to also preview results in plain text files

gui/previewresultplaintext.h

@@ -0,0 +1,20 @@
+#ifndef PREVIEWRESULTPLAINTEXT_H
+#define PREVIEWRESULTPLAINTEXT_H
+#include "previewresult.h"
+
+class PreviewResultPlainText : public PreviewResult
+{
+  private:
+	QString text;
+
+  public:
+	using PreviewResult::PreviewResult;
+	PreviewResultPlainText(const PreviewResult &o);
+
+	QWidget *createPreviewWidget() override;
+	bool hasPreview() override;
+
+	void setText(QString text);
+};
+
+#endif // PREVIEWRESULTPLAINTEXT_H