Updater: Save default download path with a symbol

As we can't control how the default (temporary) path is determined,
just save a placeholder symbol to the persistent settings when using
the default path.

doomsday/engine/portable/src/updater/updatersettings.cpp

@@ -48,7 +48,21 @@ bool UpdaterSettings::isDefaultDownloadPath() const
 
 de::String UpdaterSettings::downloadPath() const
 {
-    return QSettings().value(STK_DOWNLOAD_PATH, defaultDownloadPath()).toString();
+    de::String dir = QSettings().value(STK_DOWNLOAD_PATH, defaultDownloadPath()).toString();
+    if(dir == "${DEFAULT}")
+    {
+        dir = defaultDownloadPath();
+    }
+    return dir;
+}
+
+void UpdaterSettings::setDownloadPath(de::String downloadPath)
+{
+    if(downloadPath == defaultDownloadPath())
+    {
+        downloadPath = "${DEFAULT}";
+    }
+    QSettings().setValue(STK_DOWNLOAD_PATH, downloadPath);
 }
 
 void UpdaterSettings::setFrequency(UpdaterSettings::Frequency freq)
@@ -76,11 +90,6 @@ void UpdaterSettings::setDeleteAfterUpdate(bool deleteAfter)
     QSettings().setValue(STK_DELETE, deleteAfter);
 }
 
-void UpdaterSettings::setDownloadPath(de::String downloadPath)
-{
-    QSettings().setValue(STK_DOWNLOAD_PATH, downloadPath);
-}
-
 void UpdaterSettings::useDefaultDownloadPath()
 {
     setDownloadPath(defaultDownloadPath());