Use LinkedList instead of simple Array for Node Children

[adam-rocska]

For now the children of each PlistNode in the old-style parser
are stored in a simple array. This is fine for now, but to
improve type safety, it would be better to make them linked
lists instead where the stored value is actually the first item.

This way for example with dictionaries, the next item of a key
can be enforced to always be a value, the previous item can be
anything but a key, and any non-key value can only be followed
by a key. SO the compiler will fail miserably during fuckups.