3.1: Add .app bundle config to CMakeLists.txt

[braggpd]

Add CPack/CMake macOS app bundle configuration:

if (APPLE)
  set_target_properties("Path of Building-PoE2" PROPERTIES
    MACOSX_BUNDLE TRUE
    MACOSX_BUNDLE_BUNDLE_NAME "Path of Building 2"
    MACOSX_BUNDLE_BUNDLE_VERSION "${VERSION}"
    MACOSX_BUNDLE_SHORT_VERSION_STRING "${VERSION}"
  )
  include(BundleUtilities)
endif()

• .app bundle produced by cmake --install
• All dylibs bundled inside Contents/Frameworks/
• App double-clicks and launches

See MACOS_PORT.md §Phase 3 · 3.1