Previously, the code was not able to detect that an element overlaps on
its end, nor could it detect that an element overlaps two elements that
already overlap.
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
This is implemented on top of a Tree that represents the whole timeline.
SourceClips can not fully overlap anymore and the tests have been
updated to take that into account. Some new tests were added to verify
that behaviour in greater details
