Update package generator to use new APIs

* Move the toggling out from the view. The main should handle it IMO
* Moved the command registration into the main file as well in the spirit of our other packages
* Used panels
* Removed deprecations

If you have suggestions, please let me know. I want to start people out on the right foot. cc @nathansobo @kevinsawicki 
I would prefer to test the model in a precise way. Can we filter on item? Including an example integration test with the view could be good, but in this case we should be able to trust the panel API's basic functionality. It can get more complex when focus is involved.