applets/notifications: tighten internal spacings

Reason for the change

This restores the spacing seen in Plasma 6.2 and earlier, which accidentally got increased in the big code refactor for 6.3.

Also fix a code error exposed by this change: a place where we were accidentally using a layout's horizontal spacing as a vertical offset elsewhere, instead of its vertical spacing.

Test plan

Get a bunch of notifications, or make some yourself using notify-send. Make sure they have both a title and body text.

Screenshots or screen recordings

Some example notifications using the fixed spacings:

(The first two notifications don't have the icons I set, but that's a separate issue)

Bugs fixed

BUG: 449083 BUG: 448831 FIXED-IN: 6.3.4