timeline: Better handle loading inconsistent timelines

timeline: Better handle loading inconsistent timelines

Auto transition when having 3 overlapping clips in a same point in the
timeline is not supported as we can't handle it in a nice way. Before we
to avoid creating 2 overlapping transitions (which is plain broken in
NLE) were completely disabling `auto-transition` and removing all
auto-transitions in the timeline but this is pretty weird for the end
user. This commit changes and now makes sure 2 transitions are not
created in the same place.

Also cleanup previous test case.