[appmenu] Make the menu spacing more readable and consistent

Rebecca Beerequested to merge
rebeccabee/libplasma:appmenu-spacing into master

Reason for the change

Essentially, I feel that the current spacing between the menus is too cramped, and I made a personal edit to increase it to 16px, which looked nice, and after further investigation, I found that it matches the menu bars of Kirigami-based apps such as Merkuro, as well as being closer to older, Qt Widgets based apps' 20px padding between the menus in the menu bar.

Screenshots or screen recordings

Before After
image.png image.png

Reimplementation of plasma-workspace!5684 (closed)

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