Added a display now for application

I would like you to carefully watch this modification https://github.com/jsilvestre/cozy-home/compare/feature;displayname?expand=1#L8R26 since I think we need to discuss it a bit.

I made the name, description and cozy-permissions fields mandatory to install an application. If there is no displayName field, it fallbacks to the name fields to keep the backward compatibility.

I need to know if you agree with this so I can write another piece of code to properly handle the errors in UI. Otherwise I will remove the "mandatory" part of the code.