Externalize responsibility to load icons from create_menu
Externalize responsibility to load icons from create_menu

Fix the following issues:

- icons were always loaded even if another icon set was provided as
  parameter to create_menu
- setdefault was used to modify the arguments which might have had side
  effects on calling code

coldfix/udiskie,pstray/udiskie,khardix/udiskie,pstray/udiskie,coldfix/udiskie,mathstuf/udiskie