xwm: Check for WM_TRANSIENT_FOR property
xwm: Check for WM_TRANSIENT_FOR property

Don't just rely on data type.

Cloudef/wlc,Enerccio/ewlc,yohanesu75/wlc,vially/wlc,ss1h2a3tw/wlc,vially/wlc,Cloudef/wlc,yohanesu75/wlc,ss1h2a3tw/wlc,Enerccio/ewlc