Cleanup code
Disable some options by default.

Disable bline_width, falloff, link_origins and spline_origins_at_center
by default. And (dis)enable falloff when circle layer creation button is
(de)actived.
Enable bline_width when one or more of outline/advanced outline/curve gradient enabled.
Enable spline origin at center option when outline-ish layer creation is going to be created.
Enable link origins when two or more layer creation buttons is enabled.