Properly set operations basetime

For operations (especially mixers) to be able to do proper
synchronization they need the segment from different sources to be in
the same running time context.

This commit makes sure that the basetime of the incoming segments of
operations will be the same, and equal to the duration of the
already "played" time of the operation at the position at which
the new portion of the composition starts.
gnlcomposition: Don't flush_start / flush_stop according to user's flags.

We do it ourselves when needed, doing so led the pipeline to lose state,
which was updating the base time.
fixes https://bugzilla.gnome.org/show_bug.cgi?id=701146
gnlcomposition: forward non time-resetting flush stops outside the composition,
except those consecutive to a seek event.

The 'reset-time' parameter has been added in 1.0, and we need it to be FALSE
outside the composition.

Related to https://bugzilla.gnome.org/show_bug.cgi?id=701146
composition: update operations base time even when we don't update the pipeline.