Add EventCode.is_defined to determine which event codes are known
This makes it easier to determine which one of the codes actually exist and which ones are just made up because they fit below the respective REL_MAX, KEY_MAX, ...
cc @bentiss
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This makes it easier to determine which one of the codes actually exist and which ones are just made up because they fit below the respective REL_MAX, KEY_MAX, ...
cc @bentiss