Add GUI options to getdown splash screen -- user selectable channels, memory settings

The getdown splash screen is a good place for the user to interact and select which channel they want to load Jalview from, and what kind of memory settings they want (the default at the moment is 90% of physical memory).
(and a checkbox to know whether to be asked again).

These options should be remembered from session to session, and also settable from a "Launcher" tab in the Preferences window in Jalview.

Perhaps always pause at this splash screen if a JVL file is launching a new version of Jalview.