A vertical drag onto a lane whose clips overlap the dragged clip in time
used to resolve to the "nearest valid lane", which included the clip's own
lane — so a small drag snapped straight back and felt like the editor
refused the move. Overlapping clips can't share a row (a row is 1-D in
time), but the drag should still restack, never reject.
Now a conflicting onto-drop converts to an edge insertion adjacent to the
target by drag direction (up -> above, down -> below): the clip lands on
its own lane just in front of / behind the target, with its time
unchanged. Non-overlapping drops still join the lane. Deletes the
nearest-valid-placement search (~90 lines) this replaces.
Miguel Angel Simon Sierra
6b4870246c