New PurplePrefType: string with hint list

[Ethan Blanton]

Mark Doliner spake unto us the following wisdom:
> I like this, too.  Can I suggest using a GSList for the hints?  We
> rarely (never?) use the capabilities of doubly linked lists, and
> singly linked lists are slightly smaller.  It probably doesn't matter
> here, but I think it's a good idea in general.  I noticed that you use
> g_list_free_full(), which was added in glib 2.28.  Note that our
> configure.ac currently allows glib versions as far back as 2.16.0.  We
> usually do one of the following:

This is all 3.0 stuff; do we want ot hold ourselves back so far for
3.0?  Are we going to try to target (or at least achieve compatability
with) Glib/Gtk+ 3.x for libpurple 3?

Ethan