client: Make above/below consistent

The window menu entries would check for valid transients or modal dialog
window to enable or disable the above/below entries.

Yet the actual above/below function would simply check for any transient
or modal dialog, hence denying the request for fake dialogs without an
actual parent window.

Fix the logic to remain consistent, and allows fake dialogs to be placed
above or below other windows.

Signed-off-by: Olivier Fourdan <fourdan@xfce.org>
Closes:#458
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