xml-rs 0.8.0 has an `inner_mut()` method we can use instead of turning
off character escaping.

This matches how Apple's plist output works.

In order to do this without completely taking over indentation duties
ourselves, we just include the <plist> tag in the prologue, so xml-rs
doesn't know about it, and then go ahead and emit an end event for the
</plist> anyway since xml-rs doesn't particularly care.

This changes the plist writer output to more closely match Apple's plist
output. More specifically, we capitalize the encoding, emit a doctype,
and use hard tabs for indentation.