This patch renames:
* tDecimateGraphOp to tGraphSliderOp
* dgo to gso (to match with the struct rename)
* decimate_reset_bezts to reset_bezts to indicate it can be used by other functions
No functional changes
Reviewed by: Sybren A. Stüvel
Differential Revision: https://developer.blender.org/D12490
Ref: D12490