filechooserwidget: Replace 'list' page with column view

filechooserwidget: Replace 'list' page with column view

Replace the 'list' page of the main stack with another page, this
one containing a GtkColumnView. This, again, is the very minimal
code to achieve a column view - and validate the GListModel code
introduced in the previous commit - but there's a long way until
this column view covers the full range of features of the file
chooser.

The tree view still lives in an unused 'list2' page. From now on,
commits will "cannibalize" the treeview, each commit porting any
particular feature - be it a column, an event controller, etc -
to the column view, and dropping the corresponding feature from
the treeview.