Add progress (%) to the title of upload page

[pefimo]

Overview

When a large file is being uploaded, it would be very nice to let the user change tab to another one and just peek at the title to check the progress.

Explanation

Perhaps uploading a file is not a critically important activity and one doesn't want to observe the progress continuously. It is thus understandable that one might prefer to switch to another tab and do something else until the upload is finished.

Upload progress visible in tab's title would make it possible, esp. with scenarios similar to the following:

1. Upload a file to send.firefox.com.
2. Send a link via email to share it with a friend.

[johngruen]

Nice idea!

[johngruen]

this has a PR!