Activate xdg-shell, so the "minimize" button works under Tizen

We enable the build-time option "xdg-shell" for Tizen. Tested OK
with the latest Tizen Common & IVI images, it should work reliably
with upcoming snapshots & releases as we are taking care of
protocol versions compatibility.

BUG=XWALK-1382

Signed-off-by: Manuel Bachmann <manuel.bachmann@open.eurogiciel.org>