optimisation : Use g_object_newv where possible.

optimisation : Use g_object_newv where possible.

This avoids:
* triple-checking for the GType when type-checking is enabled (see #597260)
* Avoids going through an expensive no-argument checking which landed in
  glib-2.22
* Avoids going through 2 extrac functions (g_object_new -> g_object_new_valist)