Confine plugin lifecycle callbacks to UI thread

Drop the orphaned PluginCallbackList (dead
after the PluginManager migration) and hop
run_on_*_callbacks onto the UI thread via
CallAfter, snapshotting under the mutex on the
worker first. Keeps wx subscribers off the
detached load/unload workers.

tests/slic3rutils/CMakeLists.txt

@@ -3,7 +3,6 @@ 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_install.cpp
     test_plugin_lifecycle.cpp
     test_slicing_pipeline_bindings.cpp