timelinepipeline: make the caps from encoding profile writable
timelinepipeline: make the caps from encoding profile writable

https://bugzilla.gnome.org/show_bug.cgi?id=703121

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