modules/mode-line.js: Add a progress indicator to the downloads status w...

[dpapavas]

This pull request mirrors issue 485 (http://bugs.conkeror.org/issue485) and hopefully makes up for the rather badly made patches attached there.

The proposed patch adds a total progress display for active downloads to the downloads status mode-line widget. It works with both modern and pre-Downloads.jsm XULRunners and tries to present the information in as compact a form as possible.