Feedback dialog improvements

4 small improvements for the feedback dialog:

1. Feedback text is reset to emtpy string when feedback is sent
2. Attachement is reset when feedback is sent
3. The Browse button will change to Remove button when a file is attached. This allows the user to remove the attachement. Note that clicking on the input box allows the browsing at all times.
4. German translation fix

[TestLink](http://mf-geoadmin3.dev.bgdi.ch/gjn_feedback_improvements)