timedvaluecontrolsource: Use g_sequence_lookup where possible
timedvaluecontrolsource: Use g_sequence_lookup where possible

When looking for exact matches in the sequence, this results
in much simpler code than when using g_sequence_search.

https://bugzilla.gnome.org/show_bug.cgi?id=755498

centricular/gstreamer,StreamUtils/gstreamer,centricular/gstreamer,jpxiong/gstreamer,jpxiong/gstreamer,StreamUtils/gstreamer,jpxiong/gstreamer,StreamUtils/gstreamer,jpxiong/gstreamer,jpxiong/gstreamer,StreamUtils/gstreamer,centricular/gstreamer,centricular/gstreamer,StreamUtils/gstreamer,centricular/gstreamer