- don't duplicate search for nested classes in referenced project contents
- MemberLookupHelper.GetTypeInheritanceTree: use HashSet instead of List to find duplicate types
- DefaultClass.ClassInheritanceTree: cache the inheritance tree
Fixes SD2-1460 - Extreme delay requesting code completion for PropertyTabsCollection when Linq is imported
git-svn-id: svn://svn.sharpdevelop.net/sharpdevelop/branches/3.0@3573 1ccf3a8d-04fe-1044-b7c0-cef0b8235c61