Enhance favorites #839

So far, Sniffnet has been featured by a very limited "save as favorite" functionality.

First of all, such a functionality is only applicable to network hosts.
Additionally, favorite network hosts aren't saved across different sessions of the app: this means that every time Sniffnet is restarted or the analysis is interrupted, all the saved favorites are lost.

It's finally time to enhance this feature, bringing it at pace with all the other app's advancements of the past months.

Here's a list of requirements for the new, enhanced favorites:

• persist favorites across different runs of Sniffnet
• add an interactive menu showing all the saved favorites, with the possibility to remove each of them individually and add new ones
• add a new page showing traffic rate and other per-favorite statistics
• allow saving an upper layer service as favorite
• allow saving a program/process as favorite (depends on [Feature Request] Info about PID/user/program that's sending/receiving bytes. #170)
• update notifications to support the new favorites kinds