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/imagewriter.cpp

@@ -57,6 +57,11 @@
 #include <QtPlatformHeaders/QEglFSFunctions>
 #endif
 
+#ifdef Q_OS_LINUX
+#ifndef QT_NO_DBUS
+#include "linux/networkmanagerapi.h"
+#endif
+#endif
 
 ImageWriter::ImageWriter(QObject *parent)
     : QObject(parent), _repo(QUrl(QString(OSLIST_URL))), _dlnow(0), _verifynow(0),
@@ -990,11 +995,16 @@ QString ImageWriter::getPSK(const QString &ssid)
     }
 
     return psk;
+#else
+#ifndef QT_NO_DBUS
+    NetworkManagerApi nm;
+    return nm.getPSK();
 #else
     Q_UNUSED(ssid)
     return QString();
 #endif
 #endif
+#endif
 }
 
 bool ImageWriter::getBoolSetting(const QString &key)