GVariant: better support for object path arrays

Add G_VARIANT_TYPE_OBJECT_PATH_ARRAY along with accessor functions
g_variant_new_objv, g_variant_get_objv and g_variant_dup_objv.  Also add
support for '^ao' and '^a&o' format strings for g_variant_new() and
g_variant_get().

https://bugzilla.gnome.org/show_bug.cgi?id=654955