Animation: Gaussian Smooth operator for Graph Editor

Add a Gaussian smoothing operator to supersede the current
smoothing operator in the graph editor.

Advantage over the current implementation:
* Supports modal operations
* Is independent of key density
* More options in the redo panel
* More predictable Impulse Response

Option in the redo panel to change

Filter Width: How far out on each side of a key the code checks
to average key values

Sigma: The shape of the bell curve, lower values make a sharper bell curve
reducing the smoothing effect.
Too High values will make the code behave like an average filter as the
curve in the -1/1 range will almost be flat.

On a technical note, the operator needs to store additional data when running in modal
to avoid allocating/deallocating data on every modal run.
For that reason the `tGraphSliderOp` struct has been extended with
`void *operator_data` and `void (*free_operator_data)(void *operator_data)`.
The former is the data and the latter is a function responsible for freeing that data.

Pull Request: blender/blender#105635

source/blender/editors/space_graph/graph_intern.h

@@ -116,6 +116,7 @@ void GRAPH_OT_breakdown(struct wmOperatorType *ot);
 void GRAPH_OT_ease(struct wmOperatorType *ot);
 void GRAPH_OT_decimate(struct wmOperatorType *ot);
 void GRAPH_OT_blend_to_default(struct wmOperatorType *ot);
+void GRAPH_OT_gaussian_smooth(struct wmOperatorType *ot);
 void GRAPH_OT_sample(struct wmOperatorType *ot);
 void GRAPH_OT_bake(struct wmOperatorType *ot);
 void GRAPH_OT_unbake(struct wmOperatorType *ot);