Drop unused set_default-fade(float, framecnt_t) prototype
Introduce global default-fade-shape configuration variable

Some users always want the same fade in/out style, e.g., constant power,
symmetric, fast etc.

To avoid having them change the fade style manually for each fade, use a
global configuration variable instead.
Add ComboOption configuration for default-fade-shape

Let the user choose the preferred default fade shape in the editor
section of the preference menu.