Arbitrary window doesn't appear transient on X11, but a dialog does

After verifying that WM_TRANSIENT_FOR was being set correctly,
the child window was still showing up at an arbitrary place with
OpenBox as the window manager.  If it's marked as a dialog though,
it will be centered properly over the main window.

Change-Id: I0c1ae0755f51b564d986134520d036b72658fae4
Reviewed-by: Jens Bache-Wiig <jens.bache-wiig@digia.com>