group: fix priority setting

group: fix priority setting

Stop moving the group if a child clip is being edited by timeline-tree,
a child group is updating its own priority, or a layer that a clip is in
has changed priority. A group should only move if a descendant moves
layers outside of a timeline-tree edit, or the priority of the group is
set by the user.

Fixes https://gitlab.freedesktop.org/gstreamer/gst-editing-services/-/issues/89

Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-editing-services/-/merge_requests/169>