Add random scenario to the new game menu

Adds the option random scenario to the new game menu. Character is also random with profession randomly chosen from available scenario professions.
```
src/scenario.cpp:283:1: error: control may reach end of non-void function [-Werror,-Wreturn-type]
}
^
1 error generated.
make: *** [obj/scenario.o] Error 1
```

In this case, clang is actually mistaken, but only because the for loop is guaranteed to break out when `j++ == n` (note: this would be a concurrency issue in a multithreaded environment if another thread modified `_allowed_professions` after the value of `n` was decided). 

It might be better to just load `_allowed_professions` into some kind of list type and return the value at index `n` in the list. Even if you just throw away the list immediately thereafter, this should be called rarely enough to make the CPU time and memory spent effectively unnoticeable.