Refactor CMake configuration for ImGuiTestEngine and feature flags

- Updated the CMake configuration to use target_compile_definitions for ImGuiTestEngine, improving clarity and maintainability.
- Introduced IMGUI_TEST_ENGINE_DEFINITIONS to streamline the definition process for targets linking to ImGuiTestEngine.
- Modified feature flags in features.h to conditionally enable the native file dialog based on the YAZE_ENABLE_NFD flag, enhancing flexibility in feature management.

CMakeLists.txt

@@ -111,14 +111,14 @@ include(cmake/sdl2.cmake)
 # Asar
 include(cmake/asar.cmake)
 
-# ImGui
-include(cmake/imgui.cmake)
-
 # Google Test (if needed for main app integration)
 if (YAZE_BUILD_TESTS)
 include(cmake/gtest.cmake)
 endif()
 
+# ImGui (after minimal build flags are set)
+include(cmake/imgui.cmake)
+
 # Project Files
 add_subdirectory(src)