Web Inspector: Network view: add a per-file Cache tab

For http header cache debugging and performance tuning, add a "Cache" sibling tab to the Headers, Preview and Response tabs, which would list for each file:

 * if it was loaded immediately from the browser cache
 * if it first polled the server for a fresher version
 * if it ended up (re)fetching the file from the server
 * if it ended up committed to the browser cache, and for how long
 * in human language, for each of the above, why that was the case

In other words, a literal description, down to the specific combination of http headers (and their contents) passed from a potential prior request to that file, and (where applicable) in combination with the current request to the file, the mechanics implemented by the browser's cache mechanics which created that specific outcome.

Of course, any other parameters invoked that go into these decisions which I might have glossed over should also be noted; initial specs always need some evolving.