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: https://projects.blender.org/blender/blender/pulls/105635

scripts/startup/bl_ui/space_graph.py

@@ -330,6 +330,7 @@ class GRAPH_MT_slider(Menu):
         layout.operator("graph.blend_to_neighbor", text="Blend to Neighbor")
         layout.operator("graph.blend_to_default", text="Blend to Default Value")
         layout.operator("graph.ease", text="Ease")
+        layout.operator("graph.gaussian_smooth", text="Smooth")
 
 
 class GRAPH_MT_view_pie(Menu):