rewrote tag handling (due to glitch with dialogs - buttons would always be present).
tags are now updated in the message window itself, in the corresponding table row and in the sidebar (widget will also disapper if no tags are present anymore).
this also provides a proper fallback if js is not available.
improved drag&drop handling (style and helper creation).
relocated some styles (.tag and d&d helper) into css.

@krassmus: hope, you don't mind me messing with your code. :)
initially i just wanted to fix the issue from the very first line of this commit message.

re #3768


git-svn-id: svn://develop.studip.de/studip/trunk@31145 c5b85792-5220-0410-ad23-ed5b6b8ad999