GVariant: add new g_variant_new_take_string() API

Lots of people have variously asked for APIs like
g_variant_new_string_printf() in order to avoid having to use
g_strdup_printf(), create a GVariant using g_variant_new_string(), then
free the temporary string.

Instead of supporting that, plus a million other potential cases,
introduce g_variant_new_take_string() as a compromise.

It's not possible to write:

 v = g_variant_new_take_string (g_strdup_printf (....));

to get the desired result and avoid the extra copies.  In addition, it
works with many other functions.

https://bugzilla.gnome.org/show_bug.cgi?id=698455

docs/reference/glib/glib-sections.txt

@@ -3108,6 +3108,7 @@ g_variant_new_uint64
 g_variant_new_handle
 g_variant_new_double
 g_variant_new_string
+g_variant_new_take_string
 g_variant_new_object_path
 g_variant_is_object_path
 g_variant_new_signature