GVariant: add g_variant_take_ref()

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