mainwindow_showcase 教学示例：补 Designer 双版本表单（mainwindow2）+ 表单维护

- 新增 mainwindow2.{h,cpp,ui}：用 Qt Designer 生成的第二套 QMainWindow 表单
  （File/Edit/Help 菜单 + actionOpen/actionAbout_Me），供课堂对比「代码搭界面」
  与「Designer 拖界面」两条路线。
- 新增子目录 CMakeLists.txt：供单独打开该示例构建（顶层 add_subdirectory 不递归，
  与 lineedit_eye_toggle / mandelbrot_renderer 同模式）。
- mainwindow.ui / mainwindow.qrc：经 Designer 重新保存后的表单与资源清单维护
  （iconset 去掉 resource= 显式引用、属性顺序由 Designer 重排）。

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

docs/teaching/examples/mainwindow_showcase/CMakeLists.txt

@@ -0,0 +1,51 @@
+cmake_minimum_required(VERSION 3.5)
+
+project(qt_mainwindow_showcase LANGUAGES CXX)
+
+set(CMAKE_INCLUDE_CURRENT_DIR ON)
+
+set(CMAKE_AUTOUIC ON)
+set(CMAKE_AUTOMOC ON)
+set(CMAKE_AUTORCC ON)
+
+set(CMAKE_CXX_STANDARD 11)
+set(CMAKE_CXX_STANDARD_REQUIRED ON)
+
+# QtCreator supports the following variables for Android, which are identical to qmake Android variables.
+# Check http://doc.qt.io/qt-5/deployment-android.html for more information.
+# They need to be set before the find_package(Qt5 ...) call.
+
+#if(ANDROID)
+#    set(ANDROID_PACKAGE_SOURCE_DIR "${CMAKE_CURRENT_SOURCE_DIR}/android")
+#    if (ANDROID_ABI STREQUAL "armeabi-v7a")
+#        set(ANDROID_EXTRA_LIBS
+#            ${CMAKE_CURRENT_SOURCE_DIR}/path/to/libcrypto.so
+#            ${CMAKE_CURRENT_SOURCE_DIR}/path/to/libssl.so)
+#    endif()
+#endif()
+
+find_package(Qt5 COMPONENTS Widgets REQUIRED)
+
+if(ANDROID)
+  add_library(qt_mainwindow_showcase
+    main.cpp
+    mainwindow_showcase.cpp
+    mainwindow_showcase.h
+    mainwindow2.cpp
+    mainwindow2.h
+    mainwindow2.ui
+    mainwindow.ui
+  )
+else()
+add_executable(qt_mainwindow_showcase
+    main.cpp
+    mainwindow_showcase.cpp
+    mainwindow_showcase.h
+    mainwindow2.cpp
+    mainwindow2.h
+    mainwindow2.ui
+    mainwindow.ui
+    mainwindow.qrc)
+endif()
+
+target_link_libraries(qt_mainwindow_showcase PRIVATE Qt5::Widgets)