Improve automatic update process

[deanoemcke]

Due to my recent new knowledge of the wonderful 'onUpdateAvailable' chrome api event (thanks @uecasm) it should now be possible to rollout automatic updates in a non-catastrophic fashion.

The work for this is mostly done, but unfortunately it will take one more (catastrophic) update to get everybody onto a version that supports the onUpdateAvailable event.

Once on this new release (v7.x), then a subsequent available update will trigger a popup informing users of the incoming update, and offering them an option to save a backup of their tabs before continuing.

It will also automatically save a copy of the current tab session in the Session History view to help users recover tabs should anything go wrong during the update that causes tabs to disappear permanently.

[deanoemcke]

The bulk of this work is in these commits:

• 2c749d9
• b8f0d0b

[Firecul]

Any update on when we will get the big chrome store update? I'm getting tired of all my tabs loading back in whenever I relaunch chrome because I'm using a developer mode extension, I can't go back to 'stock' as I'm a heavy youtube user so having to clear out the cookies costs a lot of time per day.