Add -Wconversion to compilation options.
Fix all sign-conversion warnings.

The position and duration are signed integers (gint64) in GStreamer,
but it makes more sense to use unsigned integers everywhere else in our
code. This is already the case for the durations in the playlist, and
mixing signed and unsigned integers produces several sign-conversion
warnings.

Split ondine_player_seek function into ondine_player_seek_backward and
ondine_player_seek_forward, so that we can handle unsigned values more
easily. It also simplifies the tests in each particular case.

Create ondine_player_get_position_and_duration function, to avoid
duplicated code.

Signed-off-by: Cédric Marie <cedric@hjuvi.fr.eu.org>