Always open overview when there's no open/active app

On startup (and when you close the active app) you end up with an empty screen:

This is a bit confusing as you can't do much from this state, and will inevitably have to open the overview to do something. That's why we should avoid this state, and always automatically open the overview in these cases.

In the future it will only be possible to close apps from the overview, so the case where you close the active app will go away, but there might be other cases, e.g. when the active app closes unexpectedly. In all these situations I think we should go straight to the overview.