Revert "timeline-element: Do not consider not serializable elements when getting...

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 ceb82ba3028332987d8d5251f98b4896120aa59b.

Reviewed-by: Thibault Saunier <thibault.saunier@collabora.com>
Differential Revision: https://phabricator.freedesktop.org/D627