Don't show helper text each time

[thomasballinger]

Because preferences are stored in a file separate from save games, there's currently no way to opt out of getting the "this is how to fly a ship..."-like messages of the Help system.

[thomasballinger]

There's a preferences file with everything filled out! Let's use it: https://github.com/endless-sky/endless-sky/blob/master/tests/config/preferences.txt

[thomasballinger]

People customize preferences too, so it'd be nice to save these like savegames instead of using the filled out preferences file to prevent help popups