Feature request: replace the regular `type` property in actions with a Symbol

[theorlovsky]

This would increase (1) stability and (2) security:

1. there won't be a chance to collide with a user's type property, which may be present in props
2. assuming that any object with the type property is an action is not entirely correct, but if we'd use a Symbol, that would be totally safe

I could try and open a PR if you want me to.

[theorlovsky]

@cartant any thoughts?