Service taxonomy

[GyulyVGC]

More than 6 thousand application-layer services were already identified by Sniffnet, and this PR aims to define a taxonomy of such services, associating each of them with a category.

This will make it easier to understand what an observed service is about.

The Sniffnet UI now shows a different icon for each of the defined categories, and hovering on the icon will show info about the category.

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To keep the taxonomy relevant and on point, 20 categories were defined, plus a fallback for services that aren't reasonably assignable to a specific category.

Category	Scope
Web	Web content, generic web APIs, and web transfer protocols. Examples: http, https, coap.
Email	Email delivery, access, notification, and mail-specific routing. Examples: smtp, imap, pop3.
Chat & messaging	Human messaging, chat, discussion/news, SMS and paging. Examples: irc, xmpp-client, matrix-fed.
Voice & media	Audio/video delivery, calls, conferencing, and audiovisual sessions. Examples: sip, rtsp, rtmp.
File transfer & sharing	File/object transfer, synchronization, sharing, and network filesystems. Examples: ftp, rsync, nfs, microsoft-ds.
Storage & backup	Storage infrastructure, block/tape access, backup, restore, and protected replication. Examples: iscsi, ndmp, bacula-sd.
Databases & caches	Database/query engines, data caches, database access and replication. Examples: mysql, postgresql, redis.
Remote access	Interactive computer/desktop/application access and remote execution. Examples: ssh, telnet, vnc, ms-wbt-server.
Monitoring & administration	IT monitoring, logs, diagnostics, configuration, deployment, availability, and resource management. Examples: snmp, syslog, netconf-ssh, zabbix-agent.
Naming & discovery	Name resolution and locating network devices/services. Examples: domain, mdns, llmnr, svrloc.
Network infrastructure	Address assignment, network boot, time synchronization, routing, switching, mobility, and telecom/network control. Examples: dhcps, ntp, bgp, vxlan.
VPN, tunnels & proxies	General connection tunneling, network relays, VPN negotiation, and proxies. Examples: openvpn, isakmp, socks, http-proxy.
Identity & directory	Authentication, authorization, directories, identity lookup, and credential/key/certificate services. Examples: kerberos, ldap, radius, kmip.
Printing & scanning	Printing, document scanning, fax, and their dedicated peripheral protocols. Examples: ipp, printer, jetdirect, sane-port.
Gaming	Game sessions, multiplayer transports, game lobbies, and game-specific communication. Examples: minecraft, quake3, xbox.
Industrial & device automation	Physical process/equipment control, building/home automation, industrial telemetry and metering. Examples: mbap, bacnet, opcua-tcp, zigbee-ip.
Software development	Source control, compilation/build/test tools, debugging, and development support. Examples: git, svn, distcc, gdbremote.
Application middleware	General application messaging, RPC, distributed objects, transaction coordination and shared application infrastructure. Examples: amqp, mqtt, msrpc, rtps-dd-ut.
Security tools	Threat/vulnerability detection, prevention, integrity checking, security policy enforcement and incident exchange. Examples: nessus, spamassassin, prelude, idxp.
Software licensing	Software license distribution, entitlement verification, activation, and floating-license services. Examples: flexlm, CodeMeter, WibuKey, keysrvr.
Other	Single fallback for specialized applications outside this taxonomy, ambiguous identities, or insufficiently documented purposes. Examples: dicom, bitcoin, NFS-or-IIS.

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I made category icons with Inkscape to be lightweight SVGs that are visible and recognisable both on dark and light backgrounds.

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Fixes #1102