"Clear completed" button should not be active when there is (are) yet no completed file(s)

[Nokia808]

Currently "Clear completed" button is always active even if there is no any file(s) completely downloaded. This not suitable. It should be active only when there is something really completed in download section.

I test this issue on version 0.4.2 on Fedora Linux.

Please, update source code here before push it to Firefox add-on page.