resolve aliases when browsing in the 3rd pane (by type). Fixes #1484

Does what it says on the tin.
I created a new method for file objects to easily get the resolved object of an alias object. It can be used 'blindly' to resolve objects (i.e. if the object isn't an alias then calling `[anObj resolvedAliasObject]` just returns `anObj`

I contemplated changing `isDirectory` and `isFolder` to look at the resolved object (since they fail for file objects that are aliases to folders), but thought that it would be better to let the developer make the decision as to whether they want to distinguish folders and aliases (if not then just call `resolvedAliasObject` on the object)