Clipboard: Immediately retain mime data in client

Instead of synchronously grabbing clipboard data when it is needed,
asynchronously download the data and emit the data changed signal once new
content is available to the client. While this may lead to increased
resource usage, it makes sure the data is already within the client
when the clipboard data change signal is emitted. This has the added
benefit of caching the clipboard data client-side automatically, so the
same data is not retreived more than once.