Sets the details of the key frame at index_ inside this.
The this must already have a key frame at index_, and index_ must be smaller
than the number of key frames inside this.
| this | |
| index_ |
the index of the key frame |
| key |
the key of the key frame |
| mode |
the easing mode of the key frame |
| value |
a Value containing the value of the key frame |