filechooserwidget: Add a grid view

filechooserwidget: Add a grid view

Add a grid view outside of the widgetry tree. The grid view mimics
the column view using bindings, so we only need to manage the column
view.

Also add a button in the path bar section to toggle the view. This
is handled as a new 'toggle-view' action in the file chooser.

The way switching between views currently work is by setting either
the column or grid view as the child of the GtkScrolledWindow. This
has the benefit of unmapping the unused view, which is nice and can
avoid some tricky situations with thumbnails.

Fixes https://gitlab.gnome.org/GNOME/gtk/-/issues/233