GESTimelineObject: Properly set default duration
GESTimelineObject: Properly set default duration

Set it in the instance_init to GST_SECOND, But let the subclasses override
it.
This allows subclasses to set a different default duration

MathieuDuponchelle/PitiviGes,cfoch/gst-editing-services,MathieuDuponchelle/PitiviGes,volodymyrrudyi/gst-editing-services,volodymyrrudyi/gst-editing-services,luisbg/gst-editing-services,cfoch/ges,ahmedammar/platform_external_gst_editing_services,freesteph/gst-editing-services,cfoch/ges,MathieuDuponchelle/PitiviGes,cfoch/gst-editing-services,MathieuDuponchelle/PitiviGes,pitivi/gst-editing-services,freesteph/gst-editing-services,volodymyrrudyi/gst-editing-services,cfoch/gst-editing-services,cfoch/ges,pitivi/gst-editing-services,luisbg/gst-editing-services,freesteph/gst-editing-services,cfoch/gst-editing-services,cfoch/ges,ahmedammar/platform_external_gst_editing_services,pitivi/gst-editing-services,pitivi/gst-editing-services