display: Fix the logic for moving attached dialogs

If we have a tree of a window, a non-attached dialog, and then an
attached dialog, we want to move the second window, not the attached
dialog or the topmost. In other words, we want to move the first
non-attached window, or the first "freefloating window".

This happens in Firefox, whose Preferences dialog is freefloating,
but suboptions of those are modal dialogs.