New PurplePrefType: string with hint list

[Tomasz Wasilczyk]

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.

Please comment,
Tomek

[1] http://pastebin.com/JT1A5Yik