output: Use "base" color for background in GUI output window.

output: Use "base" color for background in GUI output window.

The "base" color in GtkStyle is used as the background color for GTK+
widgets that display editable text, e.g. GtkEntry.  That's what we're
aiming for, eventually, with the PSPP GUI output, and the default
white "base" color looks better than the default gray background
color, so use it.