New 'resource changed and not added' function

[Green21Black]

I suggest to add a new function that checks if a resource is changed but doesn't return true if it was just added.
Maybe like this:

pub fn resource_changed_existed<T>(res: change_detection::Res<T>) -> bool
where
    T: Resource,
{
    res.is_changed() && !res.is_added()
}