Linux distros that use NetworkManager: prefill WLAN PSK

Only supported when NetworkManager is used on the Linux distro
as wpa_supplicant itself does not support reading PSKs through
DBus, only setting them.

Closes #457

src/linux/networkmanagerapi.h

@@ -0,0 +1,22 @@
+#ifndef NETWORKMANAGERAPI_H
+#define NETWORKMANAGERAPI_H
+
+/*
+ * SPDX-License-Identifier: Apache-2.0
+ * Copyright (C) 2022 Raspberry Pi Ltd
+ */
+
+#include <QObject>
+
+class NetworkManagerApi : public QObject
+{
+    Q_OBJECT
+public:
+    explicit NetworkManagerApi(QObject *parent = nullptr);
+    QString getPSK();
+
+signals:
+
+};
+
+#endif // NETWORKMANAGERAPI_H