videotimecode: New GstVideoTimeCodeInterval type, ability to add to a GstVideoTimeCode

videotimecode: New GstVideoTimeCodeInterval type, ability to add to a GstVideoTimeCode

Sometimes there is a human-oriented timecode that represents an
interval between two other timecodes. It corresponds to the human
perception of "add X hours" or "add X seconds" to a specific timecode,
taking drop-frame oddities into account. This interval-representing
timecode is now a GstVideoTimeCodeInterval. Also added function to add it to
a GstVideoTimeCode.

https://bugzilla.gnome.org/show_bug.cgi?id=776447