Dive pictures: Don't enter infinity loop on invalid pictures

The recently committed refactoring of the dive-picture code introduced
a severe bug:

If an image couldn't be loaded from disk owing to an invalid file, the
filename was interpreted as an url and loaded in the background. This
succeeded, because the file actually exists. After download, the file
would then still be invalid and the whole thing restarted, leading to
an infinity loop.

To fix this, do two things:

1) Don't even try to download local files.
2) If interpreting a downloaded file fails, don't try the downloading
business again.

Signed-off-by: Berthold Stoeger <bstoeger@mail.tuwien.ac.at>

core/imagedownloader.h

@@ -46,7 +46,7 @@ signals:
 	void thumbnailChanged(QString filename, QImage thumbnail);
 private:
 	Thumbnailer();
-	void processItem(QString filename);
+	void processItem(QString filename, bool tryDownload);
 
 	mutable QMutex lock;
 	QThreadPool pool;