gadgets: simplify superclass lists of {SLIDER,{RADIO,CHECK}-BOX}-PANE
gadgets: simplify superclass lists of {SLIDER,{RADIO,CHECK}-BOX}-PANE

Mentioning BASIC-PANE as a direct superclass just complicates the
superclass graph since the respective abstract gadget class already
add BASIC-PANE. Not mentioning BASIC-PANE again is also in line with
all other concrete gadget classes.

slyrus/mcclim,slyrus/mcclim