"F-Curve Visibility" is a bad UI label for a property that only affects
the display of unselected F-Curves. This commit clarifies the property
name by making it more specific with the word "Unselected", and by using
the word "Opacity". "F-Curve" is redundant in the UI label anyway because
it is included in the panel title.
Resolves T82587
Differential Revision: https://developer.blender.org/D10027