event: add new seek parameter, "trickmode-interval"

event: add new seek parameter, "trickmode-interval"

When performing a key unit trickmode seek, it may be useful to
specify a minimum interval between the output frames, either
in very high rate cases, or as a protection against streams
that may contain an overly large amount of key frames.

One use case is ONVIF Section 6.5.3:

<https://www.onvif.org/specs/stream/ONVIF-Streaming-Spec.pdf>