Automate setup calls (canvas, coord-ratio, theme)

[marvinschmitt]

The calls

<...>
coord_fixed(ratio=1, xlim=c(0, 1), ylim=c(0, 1))+
theme_void() +
geom_simplex_canvas() <...>

are standard and should be performed behind the call API of geom_simplex and stat_simplex_density by default.

It's currently not done because the calls shall not be repeated when both geom_simplex and stat_simplex_density are used. Therefore, we need a way to see the current elements of the ggplot plot within geom_simplex and stat_simplex_density.
Alternative: Introduce flags to indicate whether the setup calls have been made.