Fix Empty/Player behaviour when using a playlist that is not the same as the Film's.

Fix Empty/Player behaviour when using a playlist that is not the same as the Film's.

Previously Empty would use the length of the film for its end point.
Now it takes a Playlist (rather than a list of Pieces) and uses the length
of that playlist for its end point.

This fixes #1543, in which single-content audio analysis jobs would run for the whole
length of the film, rather than the length of the content, producing
strange graphs and incorrect progress reports.