ges: Handle moving groups with effects inside

ges: Handle moving groups with effects inside

We were only concidering that we should let the group handle moving
transitions when changing transitions but in fact as soon as a
transition is happenning between two clips that are in a same group
the group properly handles moving the transition, so let the
group do its job.

Fixes T7543

Differential Revision: https://phabricator.freedesktop.org/D1281