Refactor custom GCode handling and mixed filament management

- Updated `custom_tool_changes` function to use filament IDs instead of extruder IDs, ensuring better compatibility with mixed filament configurations.
- Enhanced `MixedFilament` structure to include a `stable_id` for persistent identity across mixed filament entries, improving remapping during updates.
- Introduced new methods for stable ID allocation and normalization in `MixedFilamentManager`.
- Updated serialization and loading functions to maintain stable IDs, ensuring consistent behavior when mixed filament entries are modified.
- Added tests to verify the correct handling of tool changes and stable ID remapping in mixed filament scenarios.

tests/libslic3r/CMakeLists.txt

@@ -7,8 +7,10 @@ add_executable(${_TEST_NAME}_tests
 	test_clipper_offset.cpp
 	test_clipper_utils.cpp
 	test_config.cpp
+	test_custom_gcode.cpp
 	test_elephant_foot_compensation.cpp
 	test_geometry.cpp
+	test_mixed_filament.cpp
 	test_placeholder_parser.cpp
 	test_polygon.cpp
 	test_mutable_polygon.cpp