fixed bug with ignoring defined size for some main menu buttons

This is a regression bug caused by commit e17dc52b, which changed the
handling of main menu buttons for first level, last level and level
number from "simple screen buttons" to "real gadgets". Unfortunately,
it also caused ignoring their width and height, instead using the
"default button size" of 32x32 pixels for them, regardless of their
width and height defined to other values in file "graphicsinfo.conf".