Menu border-radius and bos-shadow changes

- introduce $menu_radius
- use it for menus and context-menus
- use the popover box-shadow also for menus
- use padding for menus to avoid edge overlapping
- remove the background for menus to avoid bleeding out of the round edges
2 jobs from context_menus_gtk-3-24 in 8 minutes and 19 seconds (queued for 3 seconds)
Status Job ID Name Coverage
  Build
passed #162573
build

00:08:19

manual #162574
win32 allowed to fail manual
msys2-mingw32