Use return value of type std::optional<QString> for

checkForUSBStick() and getUSBMountPath().
Add getJsonFilesOnUsbStick().

common/include/System.h

@@ -3,6 +3,10 @@
 
 
 #include <QObject>
+#include <QStringList>
+
+#include <optional>
+#include <initializer_list>
 
 class System : public QObject
 {
@@ -13,16 +17,17 @@ private:
 
     static QString errorMsg;
 
-
 public:
 
+    static const QStringList allowedJsonFiles;
 
     static bool checkForSDCard();
     static bool umountSDCard();
 
-    static bool checkForUSBStick();
-    static QString getUSBMountPath();
-    static QString getUSBDeviceName();
+    static std::optional<QString> checkForUSBStick(QString const &dirPathUnderMountPath = ".");
+    static QString getUSBMountPath(QString const &dirPathUnderMountPath = ".");
+
+    //static QString getUSBDeviceName();
     static bool umountUSBStick();
 
     static bool test_mount(const QString& dev, const QString &mountpoint);
@@ -48,7 +53,7 @@ public:
     static QString getPTU4SerialNumber();
     static QString getPTU4MACAddress();
 
-
+    static QStringList getJsonFilesOnUsbStick(QString const &mountPath);
 
 };