Moved static constant `newline' here because it should be accessible to
all FilePlayer descendants.
Return type of `played' method changed to `Object' in order to enable
returning of both strings and lists of strings.
Added constructor with BufferedImage in order to make the class
meaningfully constructible from the SequencePlayer class.
Added SequencePlayer class which displays a sequence of images separated
by a certain time interval (currently 50 ms).
Added overload for play which takes an arbitrary number of milliseconds
(of waiting between frames) as an argument. Also, default number of
milliseconds (50) assigned to a constant named `SLEEP_MS'.
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