fix(plugins): prevent callback races

Lifecycle callbacks can be registered while
another thread dispatches them, risking races
and iterator invalidation.

Snapshot shared callback objects under a mutex,
then invoke them unlocked to preserve reentrancy
and mutable callback state.

tests/slic3rutils/CMakeLists.txt

@@ -3,6 +3,7 @@ add_executable(${_TEST_NAME}_tests
     ${_TEST_NAME}_tests_main.cpp
     test_action_source.cpp
     test_plugin_host_api.cpp
+    test_plugin_callback_list.cpp
     test_plugin_capability_identifier.cpp
     test_plugin_install.cpp
     test_plugin_sort.cpp