Check for updates on a separate thread

Otherwise, if there are networking issues such as those reported at:

  https://list.nih.gov/cgi-bin/wa.exe?A2=IMAGEJ;6e3613ac.1406

Then it is impossible to use ImageJ until the update check is complete,
which might take 60 seconds or more for the network request to time out.
Rename "UpdatesAvailable" to be more accurate

The command actually only runs when we _know_ updates are in fact
available.
Merge pull request #28 from imagej/update-off-the-edt

Check for updates on a separate thread