[4.x] Add AssetCreating, AssetCreated and AssetSaving events

[ryanmitchell]

This PR adds missing events to the Asset class - creating, created and saving.

[jasonvarga]

By making this change, we can avoid adding a million mock expectations:

public function save()
{
-     $isNew = is_null(Facades\Asset::find($this->id()));
+     $isNew = is_null($this->container()->asset($this->path()));

Asset::find() does a lookup for the container which we don't need to do since we already have the container.

However AssetContainer::asset() calls Asset::make() which gives a similar mock expectation error. Changing the handful of existing mocks to partial mocks sorts that out.