GUI: Added a large set of possible output formats to the right-click menu.

To do: We still need better error handling.
- File exists: overwrite?
- Write failed popup.
- Check for empty file at the end (what if dot does not support this particular
  output format?)

gui/Gui/Scytherthread.py

@@ -23,11 +23,7 @@
 
 """ Import externals """
 import wx
-import os
-import sys
-import re
 import threading
-import StringIO
 
 #---------------------------------------------------------------------------