Bug 45553

Make FileEntry and DirectoryEntry methods accessible via Entry instance. Usually user script obtains an Entry (either file or directory) via callbacks as an 'Entry' instance, but since there's no concept of 'cast' in Javascript the script cannot call FileEntry- or DirectoryEntry-specific methods on the returned 'Entry' instance. Probably we could add some tricks in the binding code, or the quickest short-term solution would be: 1. (in idl) adding FileEntry- and DirectoryEntry-specific methods to Entry 2. (in c++ impl) making the methods virtual 3. throws an exception if Entry's base implementation is called.