Corrected icons for Modelica.Media:
- using "VariantsPackage" icon for package containing several media models 
- using "MaterialPropertiesPackage" icon for a package containing one medium

git-svn-id: https://svn.modelica.org/projects/Modelica/trunk@6860 7ce873d0-865f-4ce7-a662-4bb36ea78beb