Blender Git Loki

Fix T94191: correct (time) translation headers not showing DeltaX

Caused by {rBb0d9e6797fb8}

For the header (both Graph Editor case in general `headerTranslation` as
well as `headerTimeTranslate`) we are interested in deltas values
(not absolute values).

Since culprit commit, `snapFrameTransform` was not working with deltas
anymore, but we have to compensate for this.

For the Graph Editor, this only worked "by accident" in rB7192e57d63a5,
since `ival` is still zero at this point.

So now, reacquire the delta right after the snap operation.

Also use a more appropriate center value in the translate operator.

Maniphest Tasks: T94191

Differential Revision: https://developer.blender.org/D13641