Take a different approach for "zooming icons", imho a zoom slider is not needed.
And the only place I could imagine the zoom slider to fit in is left of the Ok/Cancel buttons
which is not possible, you can only add PushButtons there.
So, I add a "Zoom icons" checkable button (will have to evaluate if/how I make it remembers
if it is checked) that makes the icons show in 128x128 or 256x256, so if you want to see
inbeforehand how the icon will look in huge sizes, you can check this button.
But a zoom slider with multiple sizes is not needed imho. It's quicker applying the icon
and then playing around with the icon size in Dolphin rather than adding a huge code path
into the dialog.