Parent Task: T81785

The file editors/space_graph/graph_edit.c has reached ~3600 lines.
Here is a quick overview what it includes:

• set range extents, preview range
• ghosting fcurves
• keyframe manipulations (insert, copy&paste, duplicate, delete)
• the decimate modal operator
• baking and sampling (including sound)
• setting keyframe interpolation
• keyframe and cursor snapping
• mirroring keyframes
• fcurve modifiers
• drivers

This patch splits off two files

• graph_view.c: preview range, view all, view selected etc. Think about also including functions from graph_ops.c that do similar things
• move the decimate modal operator into graph_slider_ops.c. A new file suggested in the parent task.