GVariant: add g_variant_take_ref()

This function implements the following logic:

  if (g_variant_is_floating (value))
    g_variant_ref_sink (value);

which is used for consuming the return value of callbacks that may or
may not return floating references.

This patch also replaces a few instances of the above code with the new
function (GSettings, GDBus) and lifts a long-standing restriction on the
use of floating values as the return value for signal handlers by
improving g_value_take_variant().

https://bugzilla.gnome.org/show_bug.cgi?id=627974

docs/reference/glib/glib-sections.txt

@@ -2992,6 +2992,7 @@ g_variant_unref
 g_variant_ref
 g_variant_ref_sink
 g_variant_is_floating
+g_variant_take_ref
 g_variant_get_type
 g_variant_get_type_string
 g_variant_is_of_type