[WIP] needs testing:
Update the event system The main purpose of this PR was to address an issue where oUF wasn't updating pet frame's unit while switching between `pet`, `player`, and `vehicle` units while having a real pet summoned. In this sense it addresses the same issue as #481 does, but in a different way. It also addresses few other issues related to headers, e.g., unintentional registration of unit events as unitless, excessive `UpdateAllElements` calls on GRU, etc.
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