Apply mutation effects when adding them on new characters.

Fixes #9941

`player::toggle_trait` is called from chargen/tutorial and from wish.cpp, while it adds/removes the trait, it does not call the respective mutation functions that add / remove the effects.

This change makes the mutation effects visible in the chargen screen: selecting a scenario with `STR_UP_4` makes the pre-selected strength value 12, while the other stats remain at 8.

@KA101 I don't mind if you rewrite / change / edit / whatever this to fit the mutation system, you know better where and what must be done. I used `mutation_effect`/`mutation_loss_effect` as they don't print any messages.

Edit: tested with `forced_traits" : ["FLIMSY2", "STR_UP_3", "SHELL2"],` - shells are cool.