Getdown splash screen should disappear as soon as the Jalview Desktop is created/visible

The getdown splash screen (the first splash screen with the progress bar) has a timer for disappearing (if all goes well -- it doesn't disappear if Jalview fails to launch). This is to ensure the user knows something is still going on if Jalview desktop is taking a longish time to appear.
If the Jalview desktop appears very quickly then the getdown splashscreen annoyingly stays over the Jalview Desktop windows, including the "About" desktop splash screen (until that disappears too somtimes).

The getdown splashscreen is not needed once the Jalview Desktop has appeared, so it should be made to disappear.

A likely way of doing this, is monitoring the STDOUT of Jalview and looking for the "INFO: DESKTOP CREATED" message that is used by the `gradle jalviewjs` test (take out the `if (Platform.isJS())`) and interrupt the timer.