Python runtime: call "OnIdle" tasks when py_eval FBs execution queue is empty.
This is usefull to execute slow operations that can be deffered from functions called by py_eval FBs.
User python code add a callable to "OnIdle" list made available in global scope.
Overview: IDE and runtime(s)
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TODO: take content from https://hg.beremiz.org/beremiz_docs/