Change threading / blocking when loading aboutdata

When testing this I relalized that we might have races where about data is read, e.g. in the WelcomeWidget and where the custom about data is not yet loaded. So to avoid having to update all places where about data is used I would block this.

For me the backend Version check is also much more expensive and that is only needed when the user clicks on "Help about Kleopatra" so that could finish much later and is put back in a thread.