docs: Tutorial feedback and improvements

Hi, I did the tutorial a couple weeks ago for the GSoC application and I would like to give some feedback.

First, it's a very nice tutorial and the approach is well structured. I did the tutorial executing the code with a freedombox container so a couple missing imports popped up. It may be a good idea to incorporate them in the code. I'll list the bellow.

1. doc/dev/tutorial/view.rst: in __init__.py I think the import may be from plinth import menu instead of from plinth.menu import main_menu;
2. doc/dev/tutorial/components.rst: in the very first code snippet an from django.utils.translation import ugettext_lazy as _ is missing;

Another thing is that sometimes I felt a little lost about what file the code is supposed to be in. In the django tutorial they use a :caption: filename.py to indicate to which file that code belong. I think it would be a really helpful feature to include.

I guess this is it. If you agree to these, let me know and I can submit a MR with the changes.