Send an AssetEvent when modifying using get_id_mut

[DJMcNab]

Fixes #321

[ncallaway]

This change makes sense to me. I could see also updating get_with_id to call self.get in a similar way for similar reasons. I we ever decided to do more logic in get() it'd be nice to not have to also touch get_with_id().

pub fn get_with_id(&self, id: HandleId) -> Option<&T> {
  // - self.assets.get(&Handle::from_id(id))
  self.get(&Handle::from_id(id))
}

As a tangent from your change, I'm curious if you or anyone else knows why we have divergent names get_with_id and get_id_mut. I think all things being equal it'd be nice if those names were harmonized as get_with_id and get_with_id_mut or get_id and get_id_mut, but I think any changes there should be separate and discussed with @cart first.

[cart]

I think 'get_with_id()` is the better name. Another pr for that would be welcome.