gtk4: fix menu and window border

Rocket Aaronrequested to merge
rocka/breeze-gtk:fix/gtk4-menu-and-window into master

This fixes some minor but annoying issues.

  1. Black menubar top corner on non-csd windows (commit 72c0a3d0); menu shadow and highlight issues (commit 1f96f0a3). Tested with the gtk4-demo-application.

    This was implemented by adding extra 1px margin to the menu item, so it would make the menu a litter bigger in total.

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  2. Window control button overflow issues on csd titlebar (commit f57f06d2). This was first addressed in !90 before, but according to the comments, their implementation breaks Fitts' Law. I carefully tweaked the CSS values to make sure that the border of windowbuttons extend to both top and bottom of the titlebar without overflowing. Tested with the gtk4-demo and its built-in inspector.

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Edited by Rocket Aaron

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