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The messages

    RequestDiagnosticsMessage
    RequestHighlightingMessage

and

    DiagnosticsChangedMessage
    HighlightingChangedMessage

are always send/received together, so merge them into

    RequestDocumentAnnotationsMessage
    DocumentAnnotationsChangedMessage

Change-Id: I6a0b6281ed1e6efe6cb18386afe99b1d1fb58abf
Reviewed-by: Marco Bubke <marco.bubke@qt.io>
Reviewed-by: Thomas Hartmann <Thomas.Hartmann@theqtcompany.com>