New PurplePrefType: string with hint list

[Ethan Blanton]

Tomasz Wasilczyk spake unto us the following wisdom:
> I have implemented [1] suggested feature in a bit different way, than
> I was thinking before. I've just added possibility of hints
> configuration for string prefs to avoid creating not necessary types.
> By the way, I've also tied "masking" only to string prefs, as it was
> used only this way.

I like this.  It seems like a very reasonable approach.

Any other opinions?

Ethan