IconView: Fix vertical spacing

IconView: Fix header padding

We weren't setting the vertical spacing for the page's GridLayout, so it took the standard 6px causing this extra padding on top and bottom for every KCM.

The horizontal spacing is not needed, so that part can be simplified.

QWidget headers are still shorter tham QML ones, but that's pre-existing

Edit: Better apply this margins to the title widget which is only shown for iconview.

BUG: 449040 FIXED-IN: 5.24

BEFORE	AFTER