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/Forward.h

@@ -92,4 +92,6 @@ class Widget;
 class Window;
 class WindowServerConnection;
 
+enum class SortOrder;
+
 }