Use Refelection directly to access FileType, cleanup late binding code.

This resolves https://github.com/VioletGiraffe/cppcheck-vs-addin/issues/13 by using Reflection to retrieve `FileType` - dynamics binding fails for whatever reasons. This also cleans up code that uses late-binding fashion of `is` keyword by using well-known interface names. Finally this eliminates unneeded repeating retrieval of the same property.