Make it possible to sort a GTableModel by column+order.

This is accomplished by putting a GSortingProxyTableModel between the model
and the view. It's pretty simplistic but it works for this use case. :^)

Applications/ProcessManager/ProcessTableModel.h

@@ -8,6 +8,19 @@
 
 class ProcessTableModel final : public GTableModel {
 public:
+    enum Column {
+        Icon = 0,
+        Name,
+        CPU,
+        State,
+        Priority,
+        User,
+        PID,
+        Linear,
+        Physical,
+        __Count
+    };
+
     ProcessTableModel();
     virtual ~ProcessTableModel() override;