guacamole-client InputSink memory leak

InputSink attaches a keydown event listener to document to re-focus the sink field. However it doesn't check if the field is attached to the DOM, which means that if you re-create the sink and the Guacamole client, the old sink will still get keydown events and will still attempt to re-focus the old sink input field, despite it no longer being in the DOM.

Detected in 1.4, existing in later versions too.