Add USI/UCCI checkbox to Load Engine dialog

Add USI/UCCI checkbox to Load Engine dialog

Assign a default command to the -uxiAdapter when it was not yet defined,
as this will be used after ticking the checkbox, and the compile-time
default for it was an empty string, and will have found its way in
the user settings files of most users (making configuring through
the master settings file pointless).