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

gobject/gvaluetypes.c

@@ -1207,11 +1207,9 @@ g_value_set_variant (GValue   *value,
  * the ownership of the caller's reference to @variant;
  * the caller doesn't have to unref it any more (i.e. the reference
  * count of the variant is not increased).
- * 
- * It is a programmer error to pass a floating variant to this function.
- * In particular this means that callbacks in closures, and signal handlers
- * for signals of return type %G_TYPE_VARIANT, must never return floating
- * variants.
+ *
+ * If @variant was floating then its floating reference is converted to
+ * a hard reference.
  *
  * If you want the #GValue to hold its own reference to @variant, use
  * g_value_set_variant() instead.
@@ -1227,11 +1225,13 @@ g_value_take_variant (GValue   *value,
   GVariant *old_variant;
 
   g_return_if_fail (G_VALUE_HOLDS_VARIANT (value));
-  g_return_if_fail (variant == NULL || !g_variant_is_floating (variant));
 
   old_variant = value->data[0].v_pointer;
 
-  value->data[0].v_pointer = variant;
+  if (variant)
+    value->data[0].v_pointer = g_variant_take_ref (variant);
+  else
+    value->data[0].v_pointer = NULL;
 
   if (old_variant)
     g_variant_unref (old_variant);