style: Add real neutral spacing between items

Merged António Fernandes requested to merge wip/antoniof/border-spacing-in-new-views into main

We've been adding margins to items to keep them appart.

But the rubberbanding algorith doesn't account for these margins, so, e.g., when drawing a selection rectangle in the list view by starting from the space between rows, the closest row is included in the selection even if the selection extends in the opposite direction.

Now that GTK has gained support for the border-spacing CSS property in GtkListBase subclasses, which the rubberbanding algorithm accounts for, we can finally fix this bug in list view. For grid view this is not a complete fix, because items have variable sizes. But at least there is a safe space.

Resolves a large part of #2974

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