Remove finished uploads

Added a button to remove all finished uploads from the upload pane in
one go as a fix to the bug
https://bugs.freenetproject.org/view.php?id=1969

e.g. 2 finished uploads and one upload in progress

![1](https://f.cloud.github.com/assets/4464019/521490/882e36be-bfab-11e2-8ecc-315370657941.png)

After clicking on the button, the finished uploads are removed from list while not affecting upload in progress
![2](https://f.cloud.github.com/assets/4464019/521491/8a07417e-bfab-11e2-8e52-2ad9ae87953b.png)