timeline: Implement snapping to markers

This PR adds the ability to snap moving objects to markers from marker-lists present on nearby sources.

Adds a snappable flag to GESMarkerList, used to mark lists which we want to take into account during the snapping process (there can be multiple lists per source). Also implements ges_marker_list_get_closest() needed for the snapping system to pick a marker closest to a given timestamp.

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