Backport !1550 “gs-plugin-loader: Replace use of g_source_set_name_by_id()” to gnome-43
The g_source_set_name_by_id() is not thread safe and can cause a crash when the just scheduled source in a dedicated thread is already processed in the main thread. The documentation of the g_source_set_name_by_id() explicitly states this being a programmer error.
Closes #1966 (closed)
Trivial backport of !1550 (merged) to gnome-43
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