Reload or recreate web view when its process is terminated

[svara]

This PR improves handling of the terminated web view processes which leave the web views with a white screen.

@zoejessica did a thorough investigation on this topic. The following is taken from one of our PRs at 37signals.

You can read more here, but tl;dr we suspect that there are three ways web views can become blank, either:

1. they are killed in the "normal" way in the background through memory pressure, the delegate method fires, and they can then be reloaded, or
2. they are killed by the system without triggering a delegate callback, but can be recovered with a reload, or
3. they are killed by the system in a way that leaves them non-recoverable with a reload.

This PR takes care of all three cases.

TurboNavigator needs to track the app's state (active or background) in order to decide when to reload the terminated web views. To facilitate this, I've added two public functions: appDidBecomeActive() and appDidEnterBackground().

[joemasilotti]

AMAZING! So excited for this one.

First two comments are more suggestions, feel free to ignore them if you don't agree.

But I think we need to address the last one at the end of the PR. Unless I'm missing something.

Lastly, if I have two tabs open they both seem to reload at the same time when triggering a killed web view.

[joemasilotti]

LGTM! I made one small request of moving where the internal function lives. But feel fine merging either way.