Revert "ges: timeline-pipeline: Remove playsink send_event hack"

This reverts commit 54aac450dab9ac052f2c0a913bfba5f77c1670ba.

We need this hack until we depend on gst-p-base 0.10.36
ges-timeline-pipeline: add todo to remember to remove hack

Remove playsink hack once we depend on gst-plugins-base 0.10.37
(next gst-plugins-base release)
ges-track-object: Make possible to add a track already containing a gnlobject to a track
track-object: Add method to copy a TrackObject

API: ges_track_object_copy
timelineobject: Misc API guard fixes
timelineobject: Reimplement properly the splitting method
timelineobject: Make the 'position' argument of the split method a guint64 not gin64

This makes more sense to be a guint64 as it actually is a GstClockTime,
and this way we keep the API concistent.

(This code has not been release so we can still change the API.)
tests: Add a basic test for the timeline_object_split method
    ges: Add a timeline edition mode API

        + timeline: Add a snapping-distance property
        + Bump the GLib dependency to 2.28 in the mean time as we need some functions from GSequence that only landed
	+ Update the testsuite accordingly

    API: GESTimeline:snapping-distance property
    API: ges_timeline_object_edit
    API: ges_timeline_object_ripple
    API: ges_timeline_object_ripple_end
    API: ges_timeline_object_roll_start
    API: ges_timeline_object_roll_end
    API: ges_timeline_object_trim_start
    API: ges_track_object_edit
    API: GESEdge enum
    API: GESEditMode enum
tests: Add a testsuite for the new timeline edition API
timeline: Minor documentations fixes
timelineobject: Make changing start/duration sensible to snapping

Adapt the documentation so users are aware of the behaviour

Conflicts:

	ges/ges-timeline-object.c
python: Bind the new Timeline editing mode API
timelineobject: Fix wrong naming when connection to in-point notify

inpoint -> in-point
timelineobject: Ignore notifies when needed
trackobject: Add the copy method to the API

Add documentation and plug a leak at the same time.

API: ges_track_object_copy
trackobject: Add API guards all around
trackobject: Take into account the max duration when trying to set a new duration

Change its default value to GST_CLOCK_TIME_NONE instead of 0.

(unreleased code so it still can be changed)
timeline: Rework snapping signaling so it is easier to use

API: GESTimeline::snap-started signal
API: GESTimeline::snap-ended signal

(This code has not been released so we can still change the API)
docs: Add some more docs about editing mode

Also add the documentation "Section" into ges-enum.c so the file documentation ar
 actualy taken into account in the final generated documentation.