Split some of the code of `graph_edit.c` into:
* `graph_view.c`: preview range, view all, view selected etc.
* `graph_slider_ops.c`: the decimate modal operator code.
The latter file will be extended later with more slider-based operators.
Maniphest Tasks: T81785
Reviewed By: sybren
Differential Revision: https://developer.blender.org/D9312
