Update XML representation of UIDs

[qmfrederik]

It turns out that UIDs are represented like this in XML:

<dict>
    <key>CF$UID</key>
    <integer>2</integer>
</dict>

There's not a lot of documentation, but:

• They appear in property lists used by the Swift unit tests
• Here's one blog entry describing the format
• Here's another document (see page 7) describing the format

This PR allows you to properly serialize property lists which contain UID entries.

Deserializing is not implemented; you'd manually need to detect dictionaries with CF$UID keys and convert them to UIDs.

[claunia]

Can you implement the deserializing before I publish a nuget?