kernel/object: Amend handle types to distinguish between readable and writable events

Two kernel object should absolutely never have the same handle ID type.
This can cause incorrect behavior when it comes to retrieving object
types from the handle table. In this case it allows converting a
WritableEvent into a ReadableEvent and vice-versa, which is undefined
behavior, since the object types are not the same.

This also corrects ClearEvent() to check both kernel types like the
kernel itself does.