"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