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.h

@@ -63,6 +63,8 @@ public:
 
     void set_column_hidden(int column_index, bool hidden);
 
+    void set_key_column_and_sort_order(int column, SortOrder);
+
     GUI::AbstractView& current_view()
     {
         switch (m_view_mode) {