LibGUI: Make model sorting imperative and move order to AbstractView

Instead of SortingProxyModel having a column+order, we move that state
to AbstractView. When you click on a column header, the view tells the
model to resort the relevant column with the new order.

This is implemented in SortingProxyModel by simply walking all the
reified source/proxy mappings and resorting their row indexes.

Libraries/LibGUI/MultiView.cpp

@@ -212,4 +212,11 @@ void MultiView::set_multi_select(bool multi_select)
     apply_multi_select();
 }
 
+void MultiView::set_key_column_and_sort_order(int column, SortOrder sort_order)
+{
+    for_each_view_implementation([&](auto& view) {
+        view.set_key_column_and_sort_order(column, sort_order);
+    });
+}
+
 }