[TASK] [NESTING] Guide Model must be bound to a timelineModel
Currently the guide model is owned by the KdenliveDoc, which is bound to create problems when editing multiple timelines.
The ownership needs to be transferred to TimelineModel, and the global accesses restricted as much as possible.
WIKI Project-File-Refactoring: https://invent.kde.org/multimedia/kdenlive/-/wikis/dev/ideas/Project-File-Refactoring/