Refactor download widget to split create and populate

https://bugzilla.gnome.org/show_bug.cgi?id=720941
Improve the display of remaining time

https://bugzilla.gnome.org/show_bug.cgi?id=720941
Separate the icon from the label in the download item

https://bugzilla.gnome.org/show_bug.cgi?id=720941
Make download label smaller

https://bugzilla.gnome.org/show_bug.cgi?id=720941
Fix download bar close button

https://bugzilla.gnome.org/show_bug.cgi?id=720941
Use a menu button for the download item

https://bugzilla.gnome.org/show_bug.cgi?id=720941