Experiment with heuristics for single-window hibernation criteria

[autonome]

Single window usage is by far the majority of browser users on Firefox Nightly, per the telemetry dashboard.

Let's begin exploring and experimenting with heuristics for determining whether a tab should hibernated that are more complex than time-tab-was-last-active. Some examples:

• Recency: How long ago was the tab opened? How long was the last-active session in the tab? (eg, was user just traversing tabs, or actually using the tab)

• Relevancy: Same domain as active tab? Is opener or parent of active tab?

• Adjacency: How close or far away in the tab strip is the candidate tab from tabs active in the last N minutes?

[brunoais]

Another heuristic:

• Visibility: Is the tab hidden? The longer it has been hidden, the less likely it will bother when trying to get it back.