We were only forwarding the value to the subviews, but not storing it
in m_model_column. This would cause MultiView::model_column() to return
the wrong value.
Thanks to Daniel Bos for spotting this! :^)
A MultiView is a combination of ItemView, TableView and ColumnsView
smashed into a single widget. You can switch between the view modes
by calling MultiView::set_view_mode().
Note that MultiView inherits from StackWidget, not AbstractView.
That's purely for practical reasons, although I'm not entirely sure
if there would be some benefit to having it inherit from AbstractView.