event: Require writable events for setting the running-time-offset and sequence number
event: Require writable events for setting the running-time-offset and sequence number

Otherwise multiple code paths with the same event could change the
values on each other.

https://bugzilla.gnome.org/show_bug.cgi?id=796615

StreamUtils/gstreamer,StreamUtils/gstreamer,StreamUtils/gstreamer,StreamUtils/gstreamer,StreamUtils/gstreamer