Allow user to seek to the previous/next markers onto the timeline

EDIT: We should add two keyboard shortcuts for seeking to the previous/next marker. For inspiration on how to implement it see 8d1f7644. The markers logic is in https://gitlab.gnome.org/GNOME/pitivi/-/blob/master/pitivi/timeline/markers.py.

Hi,

Neither in stable nor in master development flatpak versions I've find a way to add "navigation markers" onto the timeline.

"Navigation markers" aim at allow user to quickly navigate through the timeline. It's a bit like "seek to the first clip edge before or after the playhead" functionality except that steps are defined by the user.

The functionality can be find in current versions of Kdenlive & OpenShot for instance.

Thanks !