Show related bookmarks in BMViewDlg

And hide it if no linked bookmark exists.
BMViewDlg make twAttachedFiles small as possible
BMViewDlg: Care for twfiles hscrollbar size in resizeEvent
BMViewDlg UI fix and improve

Scrollbar policy was set wrong on the scroll area.
Added a spacer to the scroll area to keep the attached files ui on top.
BMViewDlg UI improve

Without the stretch for the vertical spacer, the twFiles had some extra
space around it in grpFiles when it was alone. the stretch=1 for
vlSqueezeTheOtherTwo fixed it.