Duplicate code completion for StateMachineBehaviour event functions

[citizenmatt]

Rider will suggest event functions for classes deriving from StateMachineBehaviour, but those event functions are already defined as virtual methods on the StateMachineBehaviour class, resulting in duplicate entries in code completion - once as an event function and again as an override to a virtual function.

The virtual methods have overloads that are not listed in the documentation as event functions, and when overridden are highlighted as having an incorrect method signature (see #876).

A simple fix would be to remove all event functions for StateMachineBehaviour, but then we'd lose things like the gutter highlight and summary tooltip. It might be better to add the overloads into api.xml (mark the final AnimationPlayableController parameter as optional) and add a flag to not suggest these, or to check for a virtual method of the same name when adding code completion (is this an expensive check?)