KIO: try to assign an icon to action submenus

[Chloe Kudryavtsev]

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).

Please find the single-line patch attached (I have little desire to make
yet another account in phabricator, especially for such a small patch,
and was encouraged to send the patch via email on irc).
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