Ark path override should try path expansion and allow ~ in the path

[DavisVaughan]

Hadley reported that he can't use ~ here and had to expand the full path manually. I'm surprised we don't try path expansion on this variable. It would be nice if we did

[lionel-]

Would also be nice to have a checkbox to enable/disable that alternative kernel path. Currently the only way to get back to the bundled Ark is to delete the path, then you have to look it up another time to reenable.