AbstractKCM: Fix separators overlaying ScrollView content

ivan tkachenko requested to merge work/ratijas/separator-overlap into master

Header and footer separators were anchored outside of their parent item, which has led to a barely-noticeable layout issue of hiding 1px of list content at the top and bottom of a page. This issue has bigger impact with thin outline background/focus style of list delegates.

Could've used a ColumnLayout, but I heard anchors are cheaper performance-wise.

CC @mart @fusionfuture

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