Value Desktop App is missing in the layoutEngineClass properties

[Alex-1984]

https://github.com/snowplow/iglu-central/blob/master/schemas/nl.basjes/yauaa_context/jsonschema/1-0-4#L67
https://yauaa.basjes.nl/expect/fieldvalues/#layoutengineclass

Different values in the schema enum and readme.
Add please Desktop App to layoutEngineClass - enum