timeline: Move all callbacks to the callback section of the file
timeline: Move all callbacks to the callback section of the file

pitivi/gst-editing-services,volodymyrrudyi/gst-editing-services,pitivi/gst-editing-services,volodymyrrudyi/gst-editing-services,pitivi/gst-editing-services,cfoch/ges,cfoch/ges,cfoch/gst-editing-services,pitivi/gst-editing-services,cfoch/gst-editing-services,cfoch/gst-editing-services,cfoch/ges,cfoch/ges,volodymyrrudyi/gst-editing-services,cfoch/gst-editing-services