Fix timeline corruption

When using getClipItemAt() at the start position of a clip, it could return the wrong clip.
For exemple it returned the clip that was ending at the frame just before (because in the scene
the end of a clip and start of the next one almost overlap).
This caused timeline corruption, for exemple when trying to delete many clips at once.
We now make sure that we get the clip we wanted. Might be caused by some changes in Qt5,
not sure it happened on Qt4.