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 
That means I need to attach the workspace to the DOM. @nathansobo curious about your perspective. Should we trust the view is doing the right thing and test the model? Or test the view directly?