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/GUndoStack.h

@@ -0,0 +1,31 @@
+#pragma once
+
+#include <AK/NonnullOwnPtrVector.h>
+#include <LibGUI/GCommand.h>
+
+class GCommand;
+
+class GUndoStack {
+public:
+    GUndoStack();
+    ~GUndoStack();
+
+    void push(NonnullOwnPtr<GCommand>&&);
+
+    bool can_undo() const { return m_stack_index < m_stack.size() && !m_stack.is_empty(); }
+    bool can_redo() const { return m_stack_index > 0 && m_stack[m_stack_index - 1].m_undo_vector.size() > 0 && !m_stack.is_empty(); }
+
+    void undo();
+    void redo();
+
+    void finalize_current_combo();
+
+private:
+    struct UndoCommandsContainer {
+        NonnullOwnPtrVector<GCommand> m_undo_vector;
+    };
+
+    NonnullOwnPtrVector<UndoCommandsContainer> m_stack;
+    int m_stack_index { 0 };
+    int m_last_updated_undo_vector_size { 0 };
+};