Better manage requests for tooltip content

[goodmami]

The tooltips for synsets and things makes a request to the server, so there is a delay between when the tooltip shows up and when the content is shown. Worse, the content from the previous entry is shown until then. Even worse, moving the mouse over several entries causes each one to be loaded sequentially, so you have to wait for each request to complete until the one you want is displayed.

To improve this:

• clear the previous tooltip contents before making the request
• cancel any in-flight requests upon issuing a new one
• put a short delay between mouse hover and tooltip display; hopefully this will be about the same amount of time for the server to respond, but the two events should be asynchronous