only set "dialog" border for transient clients if window type is unset
only set "dialog" border for transient clients if window type is unset

fix T3157

from https://specifications.freedesktop.org/wm-spec/wm-spec-latest.html :
_NET_WM_WINDOW_TYPE_DIALOG indicates that this is a dialog window.
If _NET_WM_WINDOW_TYPE is not set, then managed windows with
WM_TRANSIENT_FOR set MUST be taken as this type

rvandegrift/e,rvandegrift/e,rvandegrift/e