clip: Emit the notify::layer signal only when actually needed
clip: Emit the notify::layer signal only when actually needed

That means:
  - only when we do change layer
  - At the end of moving between two layers

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