Changes include:
- Use `IFACE_` for UI labels set in the modifier panels
- Use a sub-sub-panel for transparency
- Fix grammar and spelling mistakes
- Use more natural user-friendly wording
- Make descriptions more specific and more useful
- Don't capitalize "line art" in descriptions (tooltips)
These changes are aimed at making the UI strings more consistent with
the rest of the UI and being more helpful to someone trying to understand
how to use the modifier.
Differential Revision: https://developer.blender.org/D10750
be33d3eccd