Finally introduce the possibility to read data from a PCAP file in addition to network adapters.

This PR makes Sniffnet fully compatible with the output produced by other network analysers: this means that now you can import data from other tools and visualise it in a Sniffnet interface.

Fixes #283.

TODO list before merging:

• Add generic capture source to abstract either a network adapter or a PCAP file
• Add PCAP file selection in the UI
• Read capture timestamps instead of assigning them manually
• Change the logic of the app when importing data from a PCAP file
  • Replace the live chart with a representation that's more suitable for offline data
  • Find an alternative way to establish traffic directionality given that interface addresses are not available
  • Produce a Tick message each time 1 second elapses in the capture file metadata (made possible by Use asynchronous channels to update app state from backend #806)
  • Info column in overview page should include data about the file instead of the adapter name
  • Disable capture file export
  • Disable notifications sound
  • Update waiting screen