add xfconf knob for disabling window menu

For locked-down environments where users shall not be able to call the
window menu, add an boolean xfconf property: /general/window_menu

Signed-off-by: Enrico Weigelt, metux IT consult <info@metux.net>
2 jobs for !41 with submit/window_menu in 2 minutes and 45 seconds (queued for 4 minutes and 6 seconds)
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