#1951 - Make metadata generation more flexible (photo and movie helpers).
- added photo_get_file_metadata and movie_get_file_metadata events
- modified photo::get_file_metadata and movie::get_file_metadata to use them
- ensure that non-readable files throw exceptions
- redirected other photo metadata calls in core to photo::get_file_metadata (the helper function already exists, but in many places getimagesize is still called directly)
- added some unit tests (neither of the functions above had one)