In two cases the percentage property was actually used incorrectly, as
pointed out in T82070. The range was [0, 1], but the properties were still
displayed as percentages.
There is a preference to control whether to display factors or
percentages, so it usually doesn't make sense to manually define
properties as percentages.
Resolves T82070
Differential Revision: https://developer.blender.org/D9344
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
