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.cpp

@@ -0,0 +1,89 @@
+#include <LibGUI/GUndoStack.h>
+
+GUndoStack::GUndoStack()
+{
+}
+
+GUndoStack::~GUndoStack()
+{
+}
+
+void GUndoStack::undo()
+{
+    if (!can_undo())
+        return;
+
+    auto& undo_container = m_stack[m_stack_index];
+    auto& undo_vector = undo_container.m_undo_vector;
+
+    //If we try to undo a empty vector, delete it and skip over.
+    if (undo_vector.is_empty()) {
+        m_stack.remove(m_stack_index);
+        undo();
+        return;
+    }
+
+    for (int i = 0; i < undo_vector.size(); i++) {
+        auto& undo_command = undo_vector[i];
+        undo_command.undo();
+    }
+
+    m_stack_index++;
+}
+
+void GUndoStack::redo()
+{
+    if (!can_redo())
+        return;
+
+    auto& undo_container = m_stack[m_stack_index - 1];
+    auto& redo_vector = undo_container.m_undo_vector;
+
+    for (int i = redo_vector.size() - 1; i >= 0; i--) {
+        auto& undo_command = redo_vector[i];
+        undo_command.redo();
+    }
+
+    m_stack_index--;
+}
+
+void GUndoStack::push(NonnullOwnPtr<GCommand>&& command)
+{
+    if (m_stack.is_empty()) {
+        auto undo_commands_container = make<UndoCommandsContainer>();
+        m_stack.prepend(move(undo_commands_container));
+    }
+
+    // Clear the elements of the stack before the m_undo_stack_index (Excluding our new element)
+    for (int i = 1; i < m_stack_index; i++)
+        m_stack.remove(1);
+
+    if (m_stack_index > 0 && !m_stack.is_empty())
+        m_stack[0].m_undo_vector.clear();
+
+    m_stack_index = 0;
+
+    m_stack[0].m_undo_vector.prepend(move(command));
+}
+
+
+void GUndoStack::finalize_current_combo()
+{
+    if (m_stack.is_empty())
+        return;
+
+    auto& undo_vector = m_stack[0].m_undo_vector;
+
+    if (undo_vector.size() == m_last_updated_undo_vector_size && !undo_vector.is_empty()) {
+        auto undo_commands_container = make<UndoCommandsContainer>();
+        m_stack.prepend(move(undo_commands_container));
+        // Note: Remove dbg() if we're 100% sure there are no bugs left.
+        dbg() << "Undo stack increased to " << m_stack.size();
+
+        // Shift the index to the left since we're adding an empty container.
+        if (m_stack_index > 0)
+            m_stack_index++;
+    }
+
+    m_last_updated_undo_vector_size = undo_vector.size();
+}