Fixed `PopUpList` widget to display correctly non-string objects ...using their `#displayOn:x:y:` / or `#displayString`. The popup menu shown the `#displayString` correctly however the "button" stubbornly used object's `#printString`, making the widget largely unusable with anything but strings. This commit removes this limitation.

LIBRARY         libwidg
DESCRIPTION     'Smalltalk/X Widget Classes'
VERSION         3.5
CODE            EXECUTE READ 
DATA            READ WRITE
SECTIONS
    INITCODE    READ EXECUTE 
    INITDATA    READ WRITE
    RODATA      READ SHARED
EXPORTS 
    _libwidg_Init       @1