- 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)
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