Move ImageDownloader out of the desktop widgets

This required a bit more untangling, but with this it seems we can build
subsurface-mobile again (at least on the desktop).

Interesting is the removal from inside the ImageDownloader of the call to
DivePictureModel::instance()->updateDivePictures() - which actually could
cause some interesting recursion issues. If it turns out we did indeed
need this, it needs to be re-architected.

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>

subsurface-core/imagedownloader.h

@@ -0,0 +1,29 @@
+#ifndef IMAGEDOWNLOADER_H
+#define IMAGEDOWNLOADER_H
+
+#include <QImage>
+#include <QFuture>
+#include <QNetworkReply>
+
+typedef QPair<QString, QByteArray> SHashedFilename;
+
+class ImageDownloader : public QObject {
+	Q_OBJECT;
+public:
+	ImageDownloader(struct picture *picture);
+	void load();
+
+private:
+	struct picture *picture;
+	QNetworkAccessManager manager;
+
+private slots:
+	void saveImage(QNetworkReply *reply);
+};
+
+class SHashedImage : public QImage {
+public:
+	SHashedImage(struct picture *picture);
+};
+
+#endif // IMAGEDOWNLOADER_H