Use G_TYPE_PTR_ARRAY for GPtrArray-of-NMObject-valued properties,
because it has better introspection/bindings support.

As with the strdict change in libnm-core, we need to manually copy the
array in get_property() implementations, to preserve the standard
semantics that get_property() returns a copy, not the internal array.

(This patch also changes those properties so that they are always
non-NULL until dispose(); previously some of them could be either NULL
or 0-length at different times.)