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/AbstractView.cpp

@@ -37,7 +37,8 @@
 namespace GUI {
 
 AbstractView::AbstractView()
-    : m_selection(*this)
+    : m_sort_order(SortOrder::Ascending)
+    , m_selection(*this)
 {
 }
 
@@ -400,4 +401,13 @@ void AbstractView::set_multi_select(bool multi_select)
     }
 }
 
+void AbstractView::set_key_column_and_sort_order(int column, SortOrder sort_order)
+{
+    m_key_column = column;
+    m_sort_order = sort_order;
+
+    if (model())
+        model()->sort(column, sort_order);
+}
+
 }