filechooserwidget: Use GtkSelectionModel for selection

filechooserwidget: Use GtkSelectionModel for selection

So far, GtkFileChooserWidget has relied on GtkTreeView's selection
management. This commit moves it away from GtkTreeView, and that's
a massive surgery - sorry :(

The most important aspect of this commit is that 'selection_model'
is now the main model we deal with. Changing between directories,
recent files, and search, all sets the selection_model's model.

Selections are entirely handled by GtkSelectionModel now.