Don't call the system qtdiag to retrieve Diagnostic information

In neptune3 when opening the Debug/System Panel (bottom right corner) on the Diagnostic tab the output of qtdiag is shown. For this qtdiag from the system is called. This works fine on the embedded device as there is usually only one Qt installation. But this doesn't work on desktop systems as there doesn't have to be a Qt in the PATH at all and if there is one, it doesn't have to be the Qt which is used for running neptune3.

Instead the path of the currently used Qt installation should be used and checked whether a qtdiag binary exists and only then the output should be listed.