LibGUI: Show command name in GUI::TextEditor undo/redo action text

We can now show things like "Undo Insert Text" and "Redo Remove Text"
instead of just "Undo" and "Redo" in menu items. Pretty neat! :^)

Userland/Libraries/LibGUI/TextDocument.cpp

@@ -719,6 +719,11 @@ InsertTextCommand::InsertTextCommand(TextDocument& document, const String& text,
 {
 }
 
+String InsertTextCommand::action_text() const
+{
+    return "Insert Text";
+}
+
 bool InsertTextCommand::merge_with(GUI::Command const& other)
 {
     if (!is<InsertTextCommand>(other))
@@ -804,6 +809,11 @@ RemoveTextCommand::RemoveTextCommand(TextDocument& document, const String& text,
 {
 }
 
+String RemoveTextCommand::action_text() const
+{
+    return "Remove Text";
+}
+
 bool RemoveTextCommand::merge_with(GUI::Command const& other)
 {
     if (!is<RemoveTextCommand>(other))

Userland/Libraries/LibGUI/TextDocument.h

@@ -113,6 +113,8 @@ public:
     void undo();
     void redo();
 
+    UndoStack const& undo_stack() const { return m_undo_stack; }
+
     void notify_did_change();
     void set_all_cursors(const TextPosition&);
 
@@ -207,6 +209,7 @@ public:
     virtual void undo() override;
     virtual void redo() override;
     virtual bool merge_with(GUI::Command const&) override;
+    virtual String action_text() const override;
     const String& text() const { return m_text; }
     const TextRange& range() const { return m_range; }
 
@@ -222,6 +225,7 @@ public:
     virtual void redo() override;
     const TextRange& range() const { return m_range; }
     virtual bool merge_with(GUI::Command const&) override;
+    virtual String action_text() const override;
 
 private:
     String m_text;

Userland/Libraries/LibGUI/TextEditor.cpp

@@ -1636,9 +1636,22 @@ void TextEditor::document_did_change()
 
 void TextEditor::document_did_update_undo_stack()
 {
+    auto make_action_text = [](auto prefix, auto suffix) {
+        StringBuilder builder;
+        builder.append(prefix);
+        if (suffix.has_value()) {
+            builder.append(' ');
+            builder.append(suffix.value());
+        }
+        return builder.to_string();
+    };
+
     m_undo_action->set_enabled(can_undo());
     m_redo_action->set_enabled(can_redo());
 
+    m_undo_action->set_text(make_action_text("&Undo", document().undo_stack().undo_action_text()));
+    m_redo_action->set_text(make_action_text("&Redo", document().undo_stack().redo_action_text()));
+
     // FIXME: This is currently firing more often than it should.
     //        Ideally we'd only send this out when the undo stack modified state actually changes.
     if (on_modified_change)