Revert "qtui: Don't create parented dialogs on the stack"

Auto-deleting dialogs is a bad idea if we're accessing the instance
after exec(). We could work around this in the affected cases,
however it may be easier and cleaner to create the dialogs on the
stack again, but don't parent them to avoid the crashes we intended
to fix with this commit.

This reverts commit 4e452ee828fdb411e6b1db07c18e02681a30ff27.