Currently, action submenus (X-KDE-Submenu) have no icons.
This patch makes it inherit the icon of the first action.
There will always be at least one action, since empty submenus are
eliminated earlier in the function.
If the icon of the first action is empty, it will gracefully revert to
old behavior (not having an icon).
Patch by Chloe Kudryavtsev <toast@toast.cafe>