_fill_try_list(): do not enter loops if not needed
Reduce code redundancy using intermediate variables has_front and has_back
Introduce class CoverArt
Re-order _fill_try_list arguments, place coverartobj in first position
Re-order _walk_try_list() argument, place coverartobj in first position
Re-order _caa_json_downloaded() arguments, place coverartobj first
Re-order _coverart_downloaded() arguments, place coverartobj first
Move most code from coverart() to new coverart_init()
Move class CoverArt and coverart() at end of the file
Convert most functions to class methods
Move coverart_init() code to __init__()
Convert album argument to class property
Convert metadata argument to class property
Move _coverart_http_error() to class CoverArt
Convert release argument to class property
Drop '_try_list' prefix/suffix and shorten method names
Store accepted CAA types and number of them in class properties
Tidy up, PEP8 conformance
_caa_append_image_to_trylist() -> _append_caa_image()
imagedata -> image