ges: Make setting start/duration move or trim generic

ges: Make setting start/duration move or trim generic

We were implementing the logic for moving/trimming elements specific
to SourceClip but this was not correct ass the new timeline tree allows
us to handle that for all element types in a generic and nice way.

This make us need to have groups trimming properly implemented in the
timeline tree, leading to some fixes in the group tests.

This adds tests for the various cases known to not be handled properly
by the previous code.

Fixes https://gitlab.freedesktop.org/gstreamer/gst-editing-services/issues/92