Revert "timeline-element: Do not consider not serializable elements when getting top element"
This commit was causing issue where we were reporting the toplevel element as an element but that element was actually in another not serialized group. That is very tricky to handle for end users as they are not guaranteed the toplevel clips were actually not contained in another element. This reverts commit ceb82ba3. Reviewed-by:Thibault Saunier <thibault.saunier@collabora.com> Differential Revision: https://phabricator.freedesktop.org/D627