Navigate historic data #1041

Sniffnet only allows seeing charts data related to the past 30 seconds, and traffic statistic related to the whole period of the packet capture.

There are two clear limitations with this approach:

• charts only show the most recent time-span, without giving any useful insight about past network activity
• traffic statistics cannot be filtered by time, meaning that it's a mess to find out what happened at a specific instant in the past

I believe that showing charts data related with the most recent period of time is still a good default, but we should add the possibility for users to configure which time-span to display in charts.
The selection could be made possible via a draggable and resizable window included below the main line-series chart.

This change is pretty huge since I don't want it to be limited only to charts data, but I'd like it to be extended to the whole Internet traffic statistics reported by Sniffnet: this means that configuring the charts time-span will also affect data related to network hosts, services, connections, etc... enabling users to time-travel and replay arbitrary periods of arbitrary duration up to the start of the analysis.

This requires not only changes to the UI, but also and most importantly to the backend data structures, since currently there is no association between the "when" and the "which and how much data was sent to / received from who" (we're only keeping track of the first and last instant that a given entity exchanged data for sorting purposes).