feat: support old title placeholder in title prompts

tests/test_gui.py

@@ -1501,6 +1501,7 @@ class GuiTests(TempDirMixin, unittest.TestCase):
             self.assertEqual("标题提示词", tab.title_prompt_edit.placeholderText())
             self.assertEqual("封面提示词", tab.cover_prompt_edit.placeholderText())
             self.assertEqual("保存标题提示词", tab.save_title_button.text())
+            self.assertEqual("插入旧标题", tab.insert_old_title_button.text())
             self.assertEqual("开始生成", tab.generate_button.text())
             self.assertEqual("停止", tab.stop_generate_button.text())
             self.assertEqual("重置生成结果", tab.reset_generate_button.text())
@@ -1679,6 +1680,12 @@ class GuiTests(TempDirMixin, unittest.TestCase):
             tab.title_prompt_edit.setPlainText("新标题提示词")
             tab.save_title_prompt()
             self.assertEqual("新标题提示词", prompts.load_title_prompt(title_prompt_path))
+            tab.title_prompt_edit.setPlainText("AB")
+            cursor = tab.title_prompt_edit.textCursor()
+            cursor.setPosition(1)
+            tab.title_prompt_edit.setTextCursor(cursor)
+            tab.insert_old_title_placeholder()
+            self.assertEqual("A{旧标题}B", tab.title_prompt_edit.toPlainText())
 
             tab.cover_prompt_edit.setPlainText("另存模板 {新标题}")
             with mock.patch("app.gui.QInputDialog.getText", return_value=("另存", True)):