settings: Enable either windows wrap or tiling

Both options are incompatible, make sure to allow either one or the
other, but not both.

Signed-off-by: Olivier Fourdan <fourdan@xfce.org>
Closes: #504
2 jobs for master in 3 minutes and 10 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #10244
build

00:01:17

 
  Distcheck
passed #10245
distcheck

00:01:52