src: xfce-desktop: support external command for root menu

When /desktop-menu/command is set, this will be used as external
command for showing the root menu instead of the builtin one.
(removing the entry reverts to normal behaviour)

This is useful for very customized desktops where a special
application shall be used to manage the root menu.

Signed-off-by: Enrico Weigelt, metux IT consult <>
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