gui: previewgenerator: Use QHash and guard using mutexes

gui/previewgenerator.cpp

@@ -1,20 +1,24 @@
 #include "../shared/common.h"
 #include "previewgenerator.h"
+#include <QMutexLocker>
 #include "previewgeneratorpdf.h"
 #include "previewgeneratorplaintext.h"
 #include "previewgeneratorodt.h"
 
 static PreviewGenerator *plainTextGenerator = new PreviewGeneratorPlainText();
 
-static QMap<QString, PreviewGenerator *> generators{
+static QHash<QString, PreviewGenerator *> generators{
 	{"pdf", new PreviewGeneratorPdf()}, {"txt", plainTextGenerator},  {"md", plainTextGenerator},
 	{"py", plainTextGenerator},			{"java", plainTextGenerator}, {"js", plainTextGenerator},
 	{"cpp", plainTextGenerator},		{"c", plainTextGenerator},	  {"sql", plainTextGenerator},
 	{"odt", new PreviewGeneratorOdt()}};
 
+static QMutex generatorsMutex;
 PreviewGenerator *PreviewGenerator::get(QFileInfo &info)
 {
+	QMutexLocker locker(&generatorsMutex);
 	PreviewGenerator *result = generators.value(info.suffix(), nullptr);
+	locker.unlock();
 	if(result == nullptr)
 	{
 		if(Common::isTextFile(info))