kcm: Limit the width of the scaling slider

When the screen name is too long, the combobox implicitWidth is wider than the available space, making the layout virtually equally large.

Limit the slider width to the same amount as the combobox

There's a visual change only on some of the cases, but I think is for the better. CCing @teams/vdg anyway

Depends on: frameworks/kirigami!260 (merged)

BUG: 434383 FIXED-IN: 5.22

	BEFORE	AFTER
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