Handle incorrect event handlers

Unity will only list public instance methods and property setters as event handlers. If an event handler is added and then subsequently set to `internal` or `static`, then the handler won't work correctly at run time (this needs confirmation, also needs confirming that the saved data isn't lost).

Rider should:
* Warn that the handler has incorrect access or static modifiers
* Warn if the parameters of the method don't match the parameters in the usage