Add categories for well-known services

Sniffnet supports identifying more than 6k services.
What is a service is described in the dedicated [Wiki page](https://github.com/GyulyVGC/sniffnet/wiki/Services).

Even if seeing what services are being used is generally helpful, often a service name alone doesn't tell much about its nature: for this reason I propose to add service categories to help users immediately understand what kind of traffic is being generated by each service.

Relevant service categories could be:
- Web
- Remote access
- Database
- Streaming
- Chat
- Mail
- File transfer
- VPN
- VoIP
- Game
- Cloud
- ...and possibly others

Each category should be associated with an icon to be displayed near the corresponding service.
In this way, we'll be able to keep services consistent with network hosts and programs, which already have icons associated with them.