E_STRICT fixes for filerepo stuff. Also, use 'abstract' keyword for File class,...

E_STRICT fixes for filerepo stuff.  Also, use 'abstract' keyword for File class, and mark some public methods public (and one private method protected, which I think is what was meant but no harm if private was actually meant).  Couldn't fix the non-matching declarations of Image::newFromTitle() and LocalFile::newFromTitle().