LibGUI: Add GUndoStack and GCommand classes

This patch converts the undo stack from GTextDocument into GUndoStack,
and GTextDocumentUndoCommand now inherits from GCommand.

Let's turn this into a generic mechanism that can be used to implement
undo/redo in any application. :^)

Libraries/LibGUI/GCommand.cpp

@@ -0,0 +1,5 @@
+#include <LibGUI/GCommand.h>
+
+GCommand::~GCommand()
+{
+}