GDataInputStream: Add _utf8() variants of _read_line

These will validate the resulting line, and throw a conversion error.
In practice these will likely be used by bindings, but it's good
for even C apps too that don't want to explode if that text file
they're reading into Pango actually has invalid UTF-8.

https://bugzilla.gnome.org/show_bug.cgi?id=652758