#1167 partially fixed (Inheritance and icons)
Undid the changes from r4694 and made exactly the definition as in MSL 3.2 (using "package" instead of "class"),
so this is completely backwards compatible and "extend" is such that a package inherits from a package (extending a class from a package is formally not correct Modelica 3.2; the specification rev. 2 needs to clarify this).

git-svn-id: https://svn.modelica.org/projects/Modelica/trunk@6763 7ce873d0-865f-4ce7-a662-4bb36ea78beb