| index | 要移动的关键点的索引。 |
| key | 要插入的关键点（具有其新时间）。 |
int 关键帧在移动后的索引。
移除 index 处的关键帧，然后插入关键点。
If a keyframe already exists at key.time the time of the old keyframe's position
/key[index].time/ will be used instead.
This is the desired behaviour for dragging keyframes in a curve editor.
Returns the index of the keyframe after moving it.