Add app menu svg support for aurorae
This adds support for a proper app menu icon for aurorae svg decorations. The appmenu.svg(z) file will be used for the icon if it exists. If not, it will fallback to the window's icon, the same as before.
To test it, download this theme https://f.tusooa.xyz/ArcDusk-dark-with-appmenu.tar (based on https://www.pling.com/p/1652462/ , copy keepbelow.svg into appmenu.svg ). Use it and start an application with a menu bar. Verify that the svg is displayed for the app menu button.