QML UI: do not crash on selecting dive computers

For reasons unknown to me, the DCDeviceData instance was freed way too early,
and used afterwards, obviously resulting in a SIGSEGV. This commit creates
the DCDeviceData as a direct child of the QMLManager instance, ensuring
it does not get freed prematurely.

Signed-off-by: Jan Mulder <jlmulder@xs4all.nl>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>

mobile-widgets/qmlmanager.h

@@ -18,6 +18,7 @@
 
 #include "core/btdiscovery.h"
 #include "core/gpslocation.h"
+#include "core/downloadfromdcthread.h"
 #include "qt-models/divelistmodel.h"
 
 class QMLManager : public QObject {
@@ -204,6 +205,7 @@ private:
 	bool checkDepth(DiveObjectHelper *myDive, struct dive *d, QString depth);
 	bool currentGitLocalOnly;
 	bool m_showPin;
+	DCDeviceData *m_device_data;
 
 signals:
 	void cloudUserNameChanged();