Bug 1109647 Add progress loading

Here is a simple implementation of the browser loading progress. Not sure if later down the road we should be trying to keep only one KVO each time we switch tabs? Anyways I hope this points us in the right direction.

As always I appreciate the team's thoughts.

Thanks
Since we update the progress bar only for KVO changes, the progress bar will be out-of-sync if we switch between two tabs while one of them is loading. We could fix this by updating the progress bar with the selected WebView's progress every time the selected tab changes. I'd be fine with pushing this to a follow-up.