View: Provide a clear setup path for the initial v_offset value

- View::update_scroll():
  The range for the vertical scrollbar is only updated if there
  are traces in the view.

- View::set_scroll_default():
  This new function adjusts the vertical offset of the view so
  that all traces are visible. Needs to be executed once after a
  new device was selected.

- View::signals_changed():
  We want to be able to clear stale items in-between devices, so
  we no longer want to simply return when no device is selected.
  This means we need to handle a missing device more gracefully.

  Also, we now reset the vertical scroll bar when no device is
  selected and call set_scroll_default() when needed.