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.