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.h

@@ -0,0 +1,20 @@
+#pragma once
+
+#include <AK/String.h>
+
+class GCommand {
+public:
+    virtual ~GCommand();
+
+    virtual void undo() {}
+    virtual void redo() {}
+
+    String action_text() const { return m_action_text; }
+
+protected:
+    GCommand() {}
+    void set_action_text(const String& text) { m_action_text = text; }
+
+private:
+    String m_action_text;
+};