Refactor mandelbrot const vars to setting variable

Refactor mandelbrot const vars to setting variable

Add settings (t_set set) as a member to t_session.
It is constructed as a union of settings type of each fractal, which may
prove useless if the settings for mandelbrot (t_set_man)
cover all the needs of other fractals. This seems unlikely however.
Add t_set_jul as a placeholder for the concept to be illustrative.