LibGUI: Add UndoStack::{undo,redo}_action_text()

These return the action_text() for the current undo and redo commands,
if available.

Userland/Libraries/LibGUI/UndoStack.cpp

@@ -109,4 +109,18 @@ void UndoStack::clear()
         on_state_change();
 }
 
+Optional<String> UndoStack::undo_action_text() const
+{
+    if (!can_undo())
+        return {};
+    return m_stack[m_stack_index - 1].action_text();
+}
+
+Optional<String> UndoStack::redo_action_text() const
+{
+    if (!can_redo())
+        return {};
+    return m_stack[m_stack_index].action_text();
+}
+
 }